[Dovecot] CATENATE allows empty messages

[Mike Abbott]

Something ate an important leading space from my message.

> b3 append inbox catenate (text {0+}
> text {8+}
> foobar
> )
> b3 OK [APPENDUID 1366726248 13] Append completed.

There was and should be a single space before the "text {8+}" above.