Bug 26138: Make Koha::Plugins->call return consistent value

Bug 26138: Make Koha::Plugins->call return consistent value

It must return an empty array in case the enable_plugins config flag is
disabled

Test plan:
Turn the config off in koha-conf
Start a search at the OPAC
Without this patch you got:
  Can't use string ("0") as a HASH ref while "strict refs" in use at /home/vagrant/kohaclone/opac/opac-search.pl line 661
With this patch applied you see the search result

Followed test plan - works as described.

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>