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

src/backend/utils/adt/tsquery.c

@@ -1283,10 +1283,13 @@ tsqueryrecv(PG_FUNCTION_ARGS)
 	 * Fill in the left-pointers. Checks that the tree is well-formed as a
 	 * side-effect.
 	 */
+	if (size > 0)
+	{
 		findoprnd(item, size, &needcleanup);
 
 		/* Can't have found any QI_VALSTOP nodes */
 		Assert(!needcleanup);
+	}
 
 	/* Copy operands to output struct */
 	for (uint32 i = 0; i < size; i++)