[Dovecot] CATENATE allows empty messages

Dovecot-2.2.1 allows empty messages to be APPENDed when using CATENATE: b1 append inbox catenate (text {0+}
) b1 OK [APPENDUID 1366726248 12] Append completed.

Contrast this with regular APPEND: b2 append inbox {0+} b2 NO Can't save a zero byte message.

Note that zero-size literals are OK but zero-size messages are not. So while "b1" above should fail like "b2", this should continue to succeed: b3 append inbox catenate (text {0+} text {8+} foobar ) b3 OK [APPENDUID 1366726248 13] Append completed.