Bug 14662: Add empty entries to pull downs in cataloguing form for mandatory subfields

Bug 14662: Add empty entries to pull downs in cataloguing form for mandatory subfields

First patch deals with the problem on the item form, this
patch fixes the same problem on catalouging form for the
bibliographic record.

To test:
- Adapt your frameworks:
  - Make sure 942$c is mandatory
  - Link another field to an authorised value, for example: 942$n to YESNO
- Add a new record
  - Verify that the first value of each pull down is preselected,
    there is no way to trigger the 'required' error
- Apply patch
  - Add a record
    - Verify that classification source is preselected according to the
      DefaultClassificationSource system preference
    - Verify that 942$c now defaults to empty
    - Verify all your other mandatory subfields can be set to empty
    - Verify that you can't save before correctly setting them
  - Chane your frameworks
    - Add a default value for 942$c, for example: CF
  - Add another record
    - Verify the itemtype is now preset to your default itemtype
  - Edit an existing record, verify all values show correctly

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>