Bug 28487: Fallback to default template in overdue_notices

Bug 28487: Fallback to default template in overdue_notices

There is no fallback to the "default" language if there is no
language-specific template for the lang of the patron.

I am not really sure why we are not using GetPreparredLetter here (which
defaults), but this needs to be backported into all stable branches and
so as small as possible.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>