datetime: Correct the validation of parameter in _field_format_get/_field_limit_get
datetime: Correct the validation of parameter in _field_format_get/_field_limit_get

Because field_list is an array, we should check boundary instead of NULL.

tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary