Bug 32418: Can't call method 'unblessed' on an undefined value at cataloguing/additem.pl

Bug 32418: Can't call method 'unblessed' on an undefined value at cataloguing/additem.pl

Check whether the current op is edititem or dupeitem and if so check
whether the itemnumber supplied as a query param actually exists.

If it doesn't, redirect to the additem op and hide all UI elements except
for a dialog that gives options to add a new item to the record or to
view the records holdings.

This behaviour was adapted from the addbiblio view, as suggested by
Fridolin.

To test:
1. Go to any record
2. Hit the edit button on an item in the holdings table
3. Modify the URL so that the query param for the itemnumber is either
    3.1 empty: /cgi-bin/koha/cataloguing/additem.pl?op=edititem&biblionumber=384&itemnumber=
    3.2 an itemnumber that doesn't exist:
      /cgi-bin/koha/cataloguing/additem.pl?op=edititem&biblionumber=384&itemnumber=9999999999
    3.3 whatever else you come up with..
4. Check the same thing for the dupe option (op=dupeitem)
5. Sign off

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>