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