[keyserver][lib] Allow not specifying messageID in message report responder

Summary:
We want to allow reporting E2EE messages, but without the contents.

Depends on D13459

Test Plan: Confirm that pressing "Report" on a thick thread message no longer leads to an error, and confirm that the authoritative keyserver admin gets a message that looks like this:

Reviewers: kamil, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13460