Bug 25946: borrowerRelationship can be empty

Bug 25946: borrowerRelationship can be empty

Do not show the empty dropdown with no options if borrowerRelationship
is disabled (empty) in the syspref.

To reproduce:
    1) Go to system preferences and disable borrowerRelationship.
    2) Create a new patron or modify the existing one.
    3) Under "Guarantor Information" click on "Search to add" button.
    After performing the search, select different patron to act as
    guarantor.
    4) Observe that there's empty dropdown for the relation type.
    5) Apply the patch.
    6) Repeat steps above.
    7) Observe that dropdown is no longer there.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>