Bug 14327: Fix js error "TypeError: events is null" in additem.js

Bug 14327: Fix js error "TypeError: events is null" in additem.js

If you have no item plugins, the events variable in BindPluginEvents
of additem.js will be null. So testing events.length will generate
the described error.
This patch adds a check to prevent that from happening again.

Test plan:
[1] Do not yet apply this patch !
[2] Temporarily remove framework plugins from your items (in ACQ or default
    framework). Probably you have to clear dateaccessioned.pl and
    barcode.pl.
[3] Open js console in your browser.
[4] Go to Acquisition. Open a basket and add an order from a new empty
    record.
[5] You should see js error: "TypeError: events is null" (additem.js:176)
[6] Apply this patch and reload the page (make sure that you refresh so
    that the new javascript code is read).
[7] The TypeError should be gone.
[8] Restore the framework plugins from step 2. Refresh the page again and
    verify that they still work as expected.

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>