TO_DATE: Result with a single NULL argument incompatible with Oracle

This is a corner case of MDEV-38514 (which is already a special case).

I don't think Oracle behavior is even correct, but given that a query passes on Oracle and fails on MariaDB, it's a slight incompatibility. Feel free to close as won't fix.

Upon a call of TO_DATE with a single NULL argument, Oracle does not return an error (as it does without any argument or with a single non-NULL argument), but returns NULL instead. MariaDB currently fails with a syntax error as described in MDEV-38514, and even if it's fixed, it will still be failing with a wrong number of arguments.

MariaDB [test]> SELECT TO_DATE(NULL) AS x;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') AS x' at line 1

SELECT TO_DATE(NULL) AS x;

X