populate_recordset_worker() failed to consider the possibility that the
supplied JSON data contains no rows, so that update_cached_tupdesc never
got called.  This led to a null-pointer dereference since commit 9a5e8ed28;
before that it led to a bogus "set-valued function called in context that
cannot accept a set" error.  Fix by forcing the update to happen.

Per bug #15514.  Back-patch to v11 as 9a5e8ed28 was.  (If we were excited
about the bogus error, we could perhaps go back further, but it'd take more
work to figure out how to fix it in older branches.  Given the lack of
field complaints about that aspect, I'm not excited.)

Discussion: https://postgr.es/m/15514-59d5b4c4065b178b@postgresql.org