Fix possible failure to send final transaction counts to stats collector.

Normally, we suppress sending a tabstats message to the collector unless
there were some actual table stats to send.  However, during backend exit
we should force out the message if there are any transaction commit/abort
counts to send, else the session's last few commit/abort counts will never
get reported at all.  We had logic for this, but the short-circuit test
at the top of pgstat_report_stat() ignored the "force" flag, with the
consequence that session-ending transactions that touched no database-local
tables would not get counted.  Seems to be an oversight in my commit
641912b4, which added the "force" flag.
That was back in 8.3, so back-patch to all supported versions.

src/backend/postmaster/pgstat.c

@@ -686,8 +686,8 @@ pgstat_report_stat(bool force)
 	int			i;
 
 	/* Don't expend a clock check if nothing to do */
-	if ((pgStatTabList == NULL || pgStatTabList->tsa_used == 0)
-		&& !have_function_stats)
+	if ((pgStatTabList == NULL || pgStatTabList->tsa_used == 0) &&
+		!have_function_stats && !force)
 		return;
 
 	/*