Even though it returns NULL, the MAKETIME function did not have this property set,
causing a failed assertion (designed to catch exactly this).
Fixed by setting the nullability property of MAKETIME().


mysql-test/r/func_sapdb.result:
  Bug#31160: Changed test result.
mysql-test/r/func_time.result:
  Bug#31160: Test result.
mysql-test/t/func_time.test:
  Bug#31160: Test case.
sql/item_timefunc.h:
  Bug#31160: The fix: Initializing maybe_null to true