Avoid core dump for empty prepared statement in an aborted transaction.

Brown-paper-bag bug in commit ab1f0c82: the old code here coped with
null CachedPlanSource.raw_parse_tree, the new code not so much.
Per report from Dave Cramer.

No regression test, because our core testing infrastructure doesn't
provide any easy way to exercise this path.  Fortunately, the JDBC
crew test it regularly.

Discussion: https://postgr.es/m/CADK3HH+Ug3xCysKqw_dZOnaNnytZ1Rh5yP05hjO-e4NoyRxVvA@mail.gmail.com