Enable nullability for IntermediateNodeVisitor

Author: DustinCampbell

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Intermediate/IntermediateNodeVisitor.cs

@@ -1,8 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#nullable disable
-
 namespace Microsoft.AspNetCore.Razor.Language.Intermediate;
 
 public abstract class IntermediateNodeVisitor