pgsql: Accept empty tsquery in binary input.

Accept empty tsquery in binary input.

tsqueryrecv() calls findoprnd() even when the input has no query
items, so it fails on an empty tsquery, which is legal data.
This breaks, for example, binary COPY of a table containing an
empty tsquery.

Skip the operand walk for an empty query, as parse_tsquery() does.

Bug: #19778
Reported-by: Shay Rojansky <roji(at)roji(dot)org>
Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19778-1befc1fba593ab1b@postgresql.org
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/187051f85cefa1634453c28f8fba6b11c4acef27

Modified Files
--------------
src/backend/utils/adt/tsquery.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)