Bug 26851: Don't generate a notice to library if no content

Bug 26851: Don't generate a notice to library if no content

To test:
 1 - Check out an over due to a patron
 2 - Set overdue triggers to a trigger less than number of days overdue
 3 - Make sure the patron has no email
 4 - Set content of ODUE notice to:
    [%# Comment will not print %]
 5 - perl misc/cronjobs/overdue_notices.pl
 6 - Check the message queue
 7 - There is a notice sent to the library:
    "These messages were not sent directly to the patrons."
    With no messages
 8 - Apply patch
 9 - Repeat
10 - No new message generated

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>