Scala.js: Handle casts to Null and Nothing.

[sjrd]

Directly copied from https://github.com/scala-js/scala-js/blob/06dc779548683d7106813f62ed596dd2b67827cd/compiler/src/main/scala/org/scalajs/nscplugin/GenJSCode.scala#L3362-L3369

[dottybot]

Hello, and thank you for opening this PR! 🎉

All contributors have signed the CLA, thank you! ❤️

Commit Messages

We want to keep history, but for that to actually be useful we have
some rules on how to format our commit messages (relevant xkcd).

Please stick to these guidelines for commit messages:

1. Separate subject from body with a blank line
2. When fixing an issue, start your commit message with Fix #<ISSUE-NBR>:
3. Limit the subject line to 72 characters
4. Capitalize the subject line
5. Do not end the subject line with a period
6. Use the imperative mood in the subject line ("Add" instead of "Added")
7. Wrap the body at 80 characters
8. Use the body to explain what and why vs. how

adapted from https://chris.beams.io/posts/git-commit

Have an awesome day! ☀️