Merge pull request #19834 from CyrusNajmabadi/documentSpan

Don't allow properties for a DocumentSpan to be null.

src/Features/Core/Portable/DocumentSpan.cs

@@ -35,7 +35,7 @@ public DocumentSpan(Document document, TextSpan sourceSpan)
         {
             Document = document;
             SourceSpan = sourceSpan;
-            Properties = properties;
+            Properties = properties ?? ImmutableDictionary<string, object>.Empty;
         }
 
         public override bool Equals(object obj)