BuildBoss needs to handle empty xmlns

Our project files are now a mix of empty XMLNS and official MSBuild
XMLNS. BuildBoss needs to handle both cases.

src/Tools/BuildBoss/ProjectUtil.cs

@@ -43,9 +43,9 @@ internal ProjectUtil(ProjectKey key, XDocument document)
         {
             Key = key;
             Document = document;
+            Namespace = document.Root.Name.Namespace;
             Manager = new XmlNamespaceManager(new NameTable());
-            Manager.AddNamespace("mb", SharedUtil.MSBuildNamespaceUriRaw);
-            Namespace = SharedUtil.MSBuildNamespace;
+            Manager.AddNamespace("mb", Namespace == XNamespace.None ? "" : SharedUtil.MSBuildNamespaceUriRaw);
         }
 
         internal RoslynProjectData GetRoslynProjectData()