Bug 31223: Do not explode if plugins disabled

Bug 31223: Do not explode if plugins disabled

This patch makes the code skip calculating the plugin-related task
codes => class mapping if plugins are entirely disbaled.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Plugins/BackgroundJob.t
=> FAIL: Tests explode with: Can't call method "GetPlugins" on an
undefined value at... Koha/BackgroundJob.pm line 424.
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>