Backporting from 10.4 to 10.3: MDEV-17325 NULL-ability problems with LEAST() in combination with NO_ZERO_DATE and NO_ZERO_IN_DATE

This also fixes:
MDEV-17299 Assertion `maybe_null' failed in make_sortkey

Note, during merge of the 10.1 version of MDEV-17299,
please use the 10.3 version of the code (i.e. null merge the 10.1 version).