Bug 14637: Fix add patron category under MySQL 5.7

Bug 14637: Fix add patron category under MySQL 5.7

If no dateofbirthrequired or upperagelimit is set on the interface, the
->store method will receive an empty string defined for these values.
For INT field, we must explicitely set these empty value to undef
instead to avoid MySQL 5.7 (and strict mode) to raise:
  DBD::mysql::st execute failed: Incorrect integer value: ' for column
'dateofbirthrequired''

Test plan:
Using MySQL 5.7 (and/or sql_mode=STRICT_TRANS_TABLES)
Create a patron category without specifying upperagelimit or
dateofbirthrequired

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>