MDEV-31302 Assertion `mon > 0 && mon < 13' failed in my_time_t sec_since_epoch(int, int, int, int, int, int)

The code erroneously called sec_since_epoch() for dates with zeros,
e.g. '2024-00-01'.
Fixi: adding a test that the date does not have zeros before
calling TIME_to_native().