If a patron has no email address, we need to avoid stockpiling all
their messages indefinitely.  Otherwise they get mailbombed when
their email IS added.

Note that overdues should not be affected, since the overdues job
checks whether the patron email exists before sending the message
(falling back to the admin).  The other messaging features are the
targets affected by this patch.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>