shared mailboxes: Don't assert-crash if trying to look up "" user's mailboxes.
shared mailboxes: Don't assert-crash if trying to look up "" user's mailboxes.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot