Bug 17257: Fix add/edit patrons under MySQL 5.7

Bug 17257: Fix add/edit patrons under MySQL 5.7

If no guarantor is defined the patron won't be added/modified and an
error will be raised:

DBD::mysql::st execute failed: Incorrect integer value: '' for column
'guarantorid'

Test plan:
Using MySQL 5.7 (and/or sql_mode=STRICT_TRANS_TABLES)
Create a patron without guarantor

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Tested with MariaDB 10.0.30 (sql_mode='STRICT_TRANS_TABLES')
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>