check clipboard payload for null

We should check that the payload is not null before setting the clipboard, since otherwise we nuke the clipboard contents.

Author: mavaddat

src/TableView.cs

@@ -418,13 +418,20 @@ internal void CopyToClipboardInternal(bool includeHeaders)
         {
             return;
         }
+        
+        var content = GetSelectedClipboardContent(includeHeaders);
 
-        var package = new DataPackage();
-        package.SetText(GetSelectedClipboardContent(includeHeaders));
+        if (string.IsNullOrWhiteSpace(content))
+        {
+            return;
+        }
 
         // Try/catch to prevent CLIPBRD_E_CANT_OPEN crashes.
         try
         {
+            var package = new DataPackage();
+            package.SetText(content);
+            
             Clipboard.SetContent(package);
         }
         catch (Exception ex)