postgres_fdw: Useless test in pgfdw_exec_cleanup_query_end()

Another thing I noticed while working on [1] is $SUBJECT: this
function checks whether the given query string is non-NULL or not when
creating a WARNING message, but the function is always called with the
query string set, so it should be non-NULL.  I removed the check and
instead added an assertion ensuring that the query string is non-NULL.
(I added the assertion to pgfdw_exec_cleanup_query_begin() as well.)
Attached is a patch for that.

If there are no objections, I will apply the patch to HEAD only.

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CAPmGK15DF6EE7O6hTLbe5-fHvPDwEx9vm-BOCN3dsKOjZCo7bw%40mail.gmail.com