Bug 31364: Check length of the multiple syspref array instead whether it is null

Bug 31364: Check length of the multiple syspref array instead whether it is null

The multiple systempreferences are always defined as an empty array at
the minimum, so in order to determine whether we need to remove all
the selections from the systempreference we should check whether the
array is empty and not whether the array doesn't exist.

To test:
 1) Set at least 1 value for OPACHoldsIfAvailableAtPickupExceptions
 2) Remove all the values for OPACHoldsIfAvailableAtPickupExceptions
 and notice you get the message "Nothing to save"
 3) Apply patch and repeat, removing all the values should work now.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>