Update SA1633 for nullable reference types

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp8/DocumentationRules/SA1633CSharp8UnitTests.cs

@@ -3,9 +3,30 @@
 
 namespace StyleCop.Analyzers.Test.CSharp8.DocumentationRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using StyleCop.Analyzers.DocumentationRules;
     using StyleCop.Analyzers.Test.CSharp7.DocumentationRules;
+    using Xunit;
 
     public partial class SA1633CSharp8UnitTests : SA1633CSharp7UnitTests
     {
+        [Fact]
+        [WorkItem(3006, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3006")]
+        public async Task TestNullableDirectiveBeforeFileHeaderAsync()
+        {
+            var testCode = @"#nullable enable
+// <copyright file=""Test0.cs"" company=""FooCorp"">
+//   Copyright (c) FooCorp. All rights reserved.
+// </copyright>
+
+namespace TestNamespace
+{
+}
+";
+
+            var expected = Diagnostic(FileHeaderAnalyzers.SA1633DescriptorMissing).WithLocation(1, 1);
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers.Test/DocumentationRules/SA1633UnitTests.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Tunnel Vision Laboratories, LLC. All Rights Reserved.
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
-#nullable disable
-
 namespace StyleCop.Analyzers.Test.DocumentationRules
 {
     using System.Threading;