Don't panic for empty CCM requests.

[jhb]

Summary

A request to encrypt an empty payload without any AAD is unusual, but
it is defined behavior. Removing this assertion removes a panic and
instead returns the correct tag for an empty buffer.

Test Plan

• 'cryptocheck -d soft -va aes-ccm 0' now gets a result matching OpenSSL instead of triggering a panic

[sef]

Something made me think that it was not acceptable, but now I can't remember what it was, so it's clearly wrong.

[cem]

Yeah, I think we talked about this in one of sef@'s reviews. It might've been a later one, though.