Return null when Hover request is cancelled or no symbol details (#1316)

Previously we returned a new Hover instance without initializing the
Contents or Range members. At least in VS Code, this will result in the
LSP client attempting to access null properties. As we're not returning
any Hover data, the correct response is to simply return null.

Author: ralish

src/PowerShellEditorServices/Services/TextDocument/Handlers/HoverHandler.cs

@@ -48,7 +48,7 @@ public async Task<Hover> Handle(HoverParams request, CancellationToken cancellat
             if (cancellationToken.IsCancellationRequested)
             {
                 _logger.LogDebug("Hover request canceled for file: {0}", request.TextDocument.Uri);
-                return new Hover();
+                return null;
             }
 
             ScriptFile scriptFile = _workspaceService.GetFile(request.TextDocument.Uri);
@@ -61,7 +61,7 @@ await _symbolsService.FindSymbolDetailsAtLocationAsync(
 
             if (symbolDetails == null)
             {
-                return new Hover();
+                return null;
             }
 
             List<MarkedString> symbolInfo = new List<MarkedString>();