Bug 9016: (QA Follow-up) Remove warning from content.length test

If the letter.content is undef, the content.length test in the letter
template will trigger warning:

    Argument "" isn't numeric in numeric gt (>).

This patch eliminates that warning.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>

tools/letter.pl

@@ -187,7 +187,7 @@ sub add_form {
                 message_transport_type => $mtt,
                 is_html    => $letters->{$mtt}{is_html},
                 title      => $letters->{$mtt}{title},
-                content    => $letters->{$mtt}{content},
+                content    => $letters->{$mtt}{content}//'',
             };
         }
     }