Fix fatal when saving import mapping with a relationship but 'Primary' relationship...

Fix fatal when saving import mapping with a relationship but 'Primary' relationship type

This occurs because the correct value to save for location_type_id of Primary is '' but the current code winds up casting to 0
which won't save as it there is a foreign key involved.  Handling was previously added but only to the
(duplicate) code that deals with non-relationship fields

https://github.com/eileenmcnaughton/civicrm-core/blob/cd41fa5b5599b329bd6f54acf0a79dda85477804/CRM/Contact/Import/Form/MapField.php#L724