Uses ArgHelper in legacy RazorSyntaxTreeExtensions

Replaces explicit null checks with `ArgHelper.ThrowIfNull` in
`RazorSyntaxTreeExtensions` for conciseness.

Author: DustinCampbell

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Legacy/RazorSyntaxTreeExtensions.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
 using System.Collections.Immutable;
 
 namespace Microsoft.AspNetCore.Razor.Language.Legacy;
@@ -10,20 +9,14 @@ internal static class RazorSyntaxTreeExtensions
 {
     public static ImmutableArray<ClassifiedSpanInternal> GetClassifiedSpans(this RazorSyntaxTree syntaxTree)
     {
-        if (syntaxTree == null)
-        {
-            throw new ArgumentNullException(nameof(syntaxTree));
-        }
+        ArgHelper.ThrowIfNull(syntaxTree);
 
         return ClassifiedSpanVisitor.VisitRoot(syntaxTree);
     }
 
     public static ImmutableArray<TagHelperSpanInternal> GetTagHelperSpans(this RazorSyntaxTree syntaxTree)
     {
-        if (syntaxTree == null)
-        {
-            throw new ArgumentNullException(nameof(syntaxTree));
-        }
+        ArgHelper.ThrowIfNull(syntaxTree);
 
         return TagHelperSpanVisitor.VisitRoot(syntaxTree);
     }