[Dovecot] CATENATE allows empty messages

[Mike Abbott]

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.