postgres_fdw: Fix syntax error in fetch_attstats().

postgres_fdw: Fix syntax error in fetch_attstats().

When importing remote stats for a foreign table backed by a pre-v17
remote server, the query built/executed in this function has three NULL
placeholders for the range stats supported in v17 at the end of the
SELECT list.  Previously, it included a trailing comma after the last
NULL, like "SELECT ..., NULL, NULL, NULL, FROM pg_catalog.pg_stats ...",
causing a syntax error on the remote server.  Fix by removing the comma.

Oversight in commit 28972b6fc.

Author: Satya Narlapuram <satyanarlapuram@gmail.com>
Discussion: https://postgr.es/m/CAHg%2BQDdEE7wp1S60Fn9Kmna8KfdMo5Tu6dROLpMn_-EOUBKmWQ%40mail.gmail.com