Ref T4107. Two issues:

• With strict MySQL settings, we try to insert null into the non-nullable messageCount field. Add an initializeNew... method.
• If we don't create a new conpherence (for example, because the message body is empty), we fatal on getPHID() right now.

Also, make this stuff a little easier to test.

Used mail_handler.php to receive empty conpherence mail, and new-thread conpherence mail.