Bug 16357: Only use Log4perl middleware if appenders defined

Bug 16357: Only use Log4perl middleware if appenders defined

This patch checks that the loggers used by the middleware
actually have appenders defined.

Without this patch, if the loggers don't have appenders
defined in the log4perl file, the logs will just be lost.

Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>