Summary

Check Noneness instead of Trueness in the WebSocketEndpoint.decode() method.
Fixes issue in discussion #3369

Update:

Addressed a review comment. text/bytes branches in decode() only checked key presence ("text" not in message), not the value.

ASGI spec says:

Exactly one of bytes or text must be non-None. One or both keys may be present, however.

That could result in ASGI compliant message silently returning None instead of raising an error. So fixed that too.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.