Fix exclusion tests

Author: sharwell

AsyncUsageAnalyzers/AsyncUsageAnalyzers.Test/Verifiers/DiagnosticVerifier.cs

@@ -94,7 +94,7 @@ protected Task VerifyCSharpDiagnosticAsync(string source, DiagnosticResult expec
         /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
         protected Task VerifyCSharpDiagnosticAsync(string source, DiagnosticResult[] expected, CancellationToken cancellationToken, string filename = null)
         {
-            return this.VerifyDiagnosticsAsync(new[] { source }, LanguageNames.CSharp, this.GetCSharpDiagnosticAnalyzers().ToImmutableArray(), expected, cancellationToken, new[] { filename });
+            return this.VerifyDiagnosticsAsync(new[] { source }, LanguageNames.CSharp, this.GetCSharpDiagnosticAnalyzers().ToImmutableArray(), expected, cancellationToken, filename != null ? new[] { filename } : null);
         }
 
         /// <summary>