Fix null reference in C# doc comment management

src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs

@@ -281,7 +281,7 @@ private SyntaxNode GetNodeContainingTargetNode(SyntaxNode matchingNode)
                 updatedNode.IsKind(SyntaxKind.DelegateDeclaration))
             {
                 var updatedLeadingTrivia = UpdateParamTagsInLeadingTrivia(document, updatedNode, declarationSymbol, signaturePermutation);
-                if (!updatedLeadingTrivia.IsEmpty)
+                if (updatedLeadingTrivia != default && !updatedLeadingTrivia.IsEmpty)
                 {
                     updatedNode = updatedNode.WithLeadingTrivia(updatedLeadingTrivia);
                 }