Bug 35930: Add guards for plugins_enabled

The 'new' method in Koha::Plugins returns undefined if
plugins are disabled.  Therefore, calls to this method
must be guarded by a check that plugins actually are enabled.

Test plan:

* Code inspection of patch, alternatively
* Activate the ill system by installing a backend such as
  koha-illbackend-libris:
  https://github.com/Libriotech/koha-illbackend-libris
* Make sure plugins are disabled in koha-conf.xml
* In the staff interface, go to ILL requests.
* The page should load without getting an error 500.

PA amended commit message: This is not related to ILL backends being plugins or not
This is about ILL batches, where checking for metadata enrichment plugins was missing 'enable_plugins' guard
Additionally, unrelated to batches, it's also about ILLAvailability, where checking for ILL availabililty plugins was missing enable_plugins guard

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Hans Pålsson <hans.palsson@hkr.se>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Koha/Illrequest/Workflow/Availability.pm

@@ -64,6 +64,8 @@ sub get_services {
         $plugin_filter->{metadata} = $params->{metadata};
     }
 
+    return [] unless C4::Context->config("enable_plugins");
+
     my @candidates = Koha::Plugins->new()->GetPlugins($plugin_filter);
     my @services   = ();
     foreach my $plugin (@candidates) {

ill/ill-requests.pl

@@ -531,6 +531,7 @@ sub can_batch {
 
 # Get available metadata enrichment plugins
 sub get_metadata_enrichment {
+    return [] unless C4::Context->config("enable_plugins");
     my @candidates = Koha::Plugins->new()->GetPlugins({
         method => 'provides_api'
     });