When importing statistics from a remote server running PostgreSQL < 17, fetch_attstats() emits three NULL placeholders for the range statistics columns that don't exist on older servers. The string literal included a trailing comma after the last NULL, producing invalid SQL:
SELECT ... NULL, NULL, NULL, FROM pg_catalog.pg_stats ...
This fails ANALYZE or IMPORT STATISTICS on a foreign table backed by a pre-v17 remote server with a syntax error.
