Allow thread=None in Webhook.send()

[LeviPesin]

Fixes #10313

Summary

Allow thread=None in Webhook.send() by checking thread instead of thread is MISSING.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

Implicit checks like these are bad so I'm not keen on merging this as-is. See also my comment on the issue.

[LeviPesin]

I can change them to thread is not MISSING and thread is not None if you like.

[LeviPesin]

@Rapptz Should I change the checks in this way? Also, should I also add support for file=None, as it is the only argument except thread which doesn't support it?