Fix annotation on XmlElement constructor parameter (#71860)

* Fixes #71858

* Add nullable annotation to other methods that take a nullable prefix and namespace URI

* Annotate missing methed on ref XmlElement

* More annotations

src/libraries/System.Private.Xml/src/System/Xml/Dom/XmlElement.cs

@@ -37,7 +37,7 @@ internal XmlElement(XmlName name, bool empty, XmlDocument doc) : base(doc)
             }
         }
 
-        protected internal XmlElement(string prefix, string localName, string? namespaceURI, XmlDocument doc)
+        protected internal XmlElement(string? prefix, string localName, string? namespaceURI, XmlDocument doc)
         : this(doc.AddXmlName(prefix, localName, namespaceURI, null), true, doc)
         {
         }

src/libraries/System.Private.Xml/src/System/Xml/XPath/XPathNavigator.cs

@@ -1560,7 +1560,7 @@ public virtual void DeleteSelf()
             DeleteRange(this);
         }
 
-        public virtual void PrependChildElement(string prefix, string localName, string namespaceURI, string value)
+        public virtual void PrependChildElement(string? prefix, string localName, string? namespaceURI, string? value)
         {
             XmlWriter writer = PrependChild();
             writer.WriteStartElement(prefix, localName, namespaceURI);
@@ -1572,7 +1572,7 @@ public virtual void PrependChildElement(string prefix, string localName, string
             writer.Close();
         }
 
-        public virtual void AppendChildElement(string prefix, string localName, string namespaceURI, string value)
+        public virtual void AppendChildElement(string? prefix, string localName, string? namespaceURI, string? value)
         {
             XmlWriter writer = AppendChild();
             writer.WriteStartElement(prefix, localName, namespaceURI);
@@ -1584,7 +1584,7 @@ public virtual void AppendChildElement(string prefix, string localName, string n
             writer.Close();
         }
 
-        public virtual void InsertElementBefore(string prefix, string localName, string namespaceURI, string value)
+        public virtual void InsertElementBefore(string? prefix, string localName, string? namespaceURI, string? value)
         {
             XmlWriter writer = InsertBefore();
             writer.WriteStartElement(prefix, localName, namespaceURI);
@@ -1596,7 +1596,7 @@ public virtual void InsertElementBefore(string prefix, string localName, string
             writer.Close();
         }
 
-        public virtual void InsertElementAfter(string prefix, string localName, string namespaceURI, string value)
+        public virtual void InsertElementAfter(string? prefix, string localName, string? namespaceURI, string? value)
         {
             XmlWriter writer = InsertAfter();
             writer.WriteStartElement(prefix, localName, namespaceURI);
@@ -1608,7 +1608,7 @@ public virtual void InsertElementAfter(string prefix, string localName, string n
             writer.Close();
         }
 
-        public virtual void CreateAttribute(string prefix, string localName, string namespaceURI, string value)
+        public virtual void CreateAttribute(string? prefix, string localName, string? namespaceURI, string? value)
         {
             XmlWriter writer = CreateAttributes();
             writer.WriteStartAttribute(prefix, localName, namespaceURI);

src/libraries/System.Xml.ReaderWriter/ref/System.Xml.ReaderWriter.cs

@@ -398,7 +398,7 @@ public partial class XmlDocumentType : System.Xml.XmlLinkedNode
     }
     public partial class XmlElement : System.Xml.XmlLinkedNode
     {
-        protected internal XmlElement(string prefix, string localName, string? namespaceURI, System.Xml.XmlDocument doc) { }
+        protected internal XmlElement(string? prefix, string localName, string? namespaceURI, System.Xml.XmlDocument doc) { }
         public override System.Xml.XmlAttributeCollection Attributes { get { throw null; } }
         public virtual bool HasAttributes { get { throw null; } }
         public override string InnerText { get { throw null; } set { } }
@@ -2649,12 +2649,12 @@ public abstract partial class XPathNavigator : System.Xml.XPath.XPathItem, Syste
         public virtual void AppendChild(string newChild) { }
         public virtual void AppendChild(System.Xml.XmlReader newChild) { }
         public virtual void AppendChild(System.Xml.XPath.XPathNavigator newChild) { }
-        public virtual void AppendChildElement(string prefix, string localName, string namespaceURI, string value) { }
+        public virtual void AppendChildElement(string? prefix, string localName, string? namespaceURI, string? value) { }
         public virtual bool CheckValidity(System.Xml.Schema.XmlSchemaSet schemas, System.Xml.Schema.ValidationEventHandler validationEventHandler) { throw null; }
         public abstract System.Xml.XPath.XPathNavigator Clone();
         public virtual System.Xml.XmlNodeOrder ComparePosition(System.Xml.XPath.XPathNavigator? nav) { throw null; }
         public virtual System.Xml.XPath.XPathExpression Compile(string xpath) { throw null; }
-        public virtual void CreateAttribute(string prefix, string localName, string namespaceURI, string value) { }
+        public virtual void CreateAttribute(string? prefix, string localName, string? namespaceURI, string? value) { }
         public virtual System.Xml.XmlWriter CreateAttributes() { throw null; }
         public virtual System.Xml.XPath.XPathNavigator CreateNavigator() { throw null; }
         public virtual void DeleteRange(System.Xml.XPath.XPathNavigator lastSiblingToDelete) { }
@@ -2674,8 +2674,8 @@ public abstract partial class XPathNavigator : System.Xml.XPath.XPathItem, Syste
         public virtual void InsertBefore(string newSibling) { }
         public virtual void InsertBefore(System.Xml.XmlReader newSibling) { }
         public virtual void InsertBefore(System.Xml.XPath.XPathNavigator newSibling) { }
-        public virtual void InsertElementAfter(string prefix, string localName, string namespaceURI, string value) { }
-        public virtual void InsertElementBefore(string prefix, string localName, string namespaceURI, string value) { }
+        public virtual void InsertElementAfter(string? prefix, string localName, string? namespaceURI, string? value) { }
+        public virtual void InsertElementBefore(string? prefix, string localName, string? namespaceURI, string? value) { }
         public virtual bool IsDescendant([System.Diagnostics.CodeAnalysis.NotNullWhenAttribute(true)] System.Xml.XPath.XPathNavigator? nav) { throw null; }
         public abstract bool IsSamePosition(System.Xml.XPath.XPathNavigator other);
         public virtual string? LookupNamespace(string prefix) { throw null; }
@@ -2710,7 +2710,7 @@ public abstract partial class XPathNavigator : System.Xml.XPath.XPathItem, Syste
         public virtual void PrependChild(string newChild) { }
         public virtual void PrependChild(System.Xml.XmlReader newChild) { }
         public virtual void PrependChild(System.Xml.XPath.XPathNavigator newChild) { }
-        public virtual void PrependChildElement(string prefix, string localName, string namespaceURI, string value) { }
+        public virtual void PrependChildElement(string? prefix, string localName, string? namespaceURI, string? value) { }
         public virtual System.Xml.XmlReader ReadSubtree() { throw null; }
         public virtual System.Xml.XmlWriter ReplaceRange(System.Xml.XPath.XPathNavigator lastSiblingToReplace) { throw null; }
         public virtual void ReplaceSelf(string newNode) { }