MDEV-27309 Server crash or ASAN memcpy-param-overlap upon INSERT into Aria/MyISAM table with DESC key

MyiSAM and Aria, indexes with prefix compression, where the first keypart
could be NULL - in this case they didn't expect the next key after the
not NULL key to be NULL.

Expect the first keypart of the next key to have zero length even
if store_not_null==1, this combination means keypart is NULL,
don't pack it.

also fixes MDEV-27340