Bug 25567: Prevent patron category type's categorycode to be created with ""

Bug 25567: Prevent patron category type's categorycode to be created with ""

It must be NULL/undef, the default from the DB.

The method Koha::Patron::Attribute::Type->categorycode is not covered by tests!

To recreate:
Create a new patron attribute type, don't select a patron's category (and not repeatable or unique)
Go to the batch patron modification tool, enter a cardnumber, next.
=> boom!
Apply the patch, edit the patron attribute, save
Go to the batch patron modification tool, enter a cardnumber, next.
=> success!

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>