Bug 35930: Add guards for plugins_enabled

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>