C# AST: when setting a string property to null or an empty string, remove the corresponding identifier token.

ICSharpCode.NRefactory.CSharp/Ast/Identifier.cs

@@ -113,17 +113,18 @@ namespace ICSharpCode.NRefactory.CSharp
 
 		public static Identifier Create (string name, TextLocation location)
 		{
-			if (name == null)
-				throw new ArgumentNullException("name");
-			if (name.Length > 0 && name[0] == '@')
+			if (string.IsNullOrEmpty(name))
+				return Identifier.Null;
+			if (name[0] == '@')
 				return new VerbatimIdentifier(name.Substring (1), location);
-			return new Identifier (name, location);
+			else
+				return new Identifier (name, location);
 		}
 		
 		public static Identifier Create (string name, TextLocation location, bool isVerbatim)
 		{
-			if (name == null)
-				throw new ArgumentNullException("name");
+			if (string.IsNullOrEmpty(name))
+				return Identifier.Null;
 			
 			if (isVerbatim)
 				return new VerbatimIdentifier(name, location);