R1:7f45824984de

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…