The query parameter for "ref_src" when submitting "sad" feedback is "nullref_src" instead of just "ref_src" because null is erroneously concatenated.

• VSCode Version: 1.51.1 (user setup)
• OS Version: Windows_NT x64 10.0.17763

Steps to Reproduce:

1. Open the tweet feedback UI using the button in the bottom right of code.
2. Click the sad face
3. Enter some tweet content
4. Hit tweet
5. The resulting URL will be something like https://twitter.com/intent/tweet?nullref_src=twsrc%5Etfw&related=twitterapi,twitter&text=123&tw_p=tweetbutton&via=code

Does this issue occur when all extensions are disabled?: Yes