Fix database field default value test failure

The automatic upgrade process of owncloud might fail because of the default value test.

The problem is oc_share.expiration witch is DATETIME field with a default NULL value (which is logical for shares without expiration)

The MDB2 third-party app try to validate the null string as a datetime, and fails, and blocks the upgrade.

Fixed by don't test the default value is the field can be NULL.