Handle null ProjectFile in InvalidProjectFileException (#13179)

The internal InvalidProctFileException constructor accepts an existing
InvalidProjectFileException object as a parameter. It passes the
ProjectFile property from that passed in object to another constructor
that then verifies that projectFile is not null.

Whenever an InvalidProjectFileException object is passed in that has a
null projectFile (which is possible via the public ctors), an
ArgumentNullException gets thrown and hides real errors.

The object gets passed to the internal ctor here:
https://github.com/dotnet/msbuild/blob/8bd4d54903169f7e515f8a32e20190b340a295c9/src/Build/BackEnd/Components/RequestBuilder/TaskBuilder.cs#L923

Fix this by passing "MSBUILD" as a string in whenever projectFile is
null.

Fixes  #13150

Author: ViktorHofer

src/Build/Errors/InvalidProjectFileException.cs

@@ -71,7 +71,7 @@ public InvalidProjectFileException(string message, Exception innerException)
         /// The assumption is that all the metadata for the outer exception comes from the inner exception, eg., they have the same error code.
         /// </summary>
         internal InvalidProjectFileException(string message, InvalidProjectFileException innerException)
-            : this(innerException.ProjectFile, innerException.LineNumber, innerException.ColumnNumber, innerException.EndLineNumber, innerException.EndColumnNumber, message, innerException.ErrorSubcategory, innerException.ErrorCode, innerException.HelpKeyword)
+            : this(innerException.ProjectFile ?? "MSBUILD", innerException.LineNumber, innerException.ColumnNumber, innerException.EndLineNumber, innerException.EndColumnNumber, message, innerException.ErrorSubcategory, innerException.ErrorCode, innerException.HelpKeyword)
         {
         }