Cleanup and tighten Letters module.

Cleanup and tighten Letters module.

Subroutine arguments enforced w/ with more checks, explicit return undef where warranted.
Placeholders used in SQL where applicable.
One logical error corrected :
- next MESSAGE if ( lc( $message->{'message_transport_type'} eq 'rss' ) );
+ next MESSAGE if ( lc($message->{'message_transport_type'}) eq 'rss' );

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>