Bug 30115: Uninitialized value warning in C4/Output.pm

Bug 30115: Uninitialized value warning in C4/Output.pm

Use of uninitialized value $url_suffix in concatenation (.) or string at GIT/C4/Output.pm line 180.
Use of uninitialized value $url_suffix in concatenation (.) or string at GIT/C4/Output.pm line 197.
Use of uninitialized value $url_suffix in concatenation (.) or string at GIT/C4/Output.pm line 211.

This is in pagination_bar sub.  A lot of pages call it.  For me it
happened with any Authority search (authorities/authorities-home.pl)
producing multiple results.

Simple fix is initializing the variable.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>