Fix two issues with creating Conpherence threads via mail on some configurations

Summary:
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.

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

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4107

Differential Revision: https://secure.phabricator.com/D7760