Fix vacuum_cost_delay check for balance calculation.

Commit 1021bd6a89 excluded autovacuum workers from cost-limit balance
calculations when per-relation options were set.  The code checks for
limit and cost_delay being greater than zero, but since cost_delay can
be set to -1 the test needs to check for greater than or zero.

Backpatch to all supported branches since 1021bd6a89 was backpatched
all the way at the time.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAD21AoBS7o6Ljt_vfqPQPf67AhzKu3fR0iqk8B=vVYczMugKMQ@mail.gmail.com
Backpatch-through: v11 (all supported branches)

src/backend/postmaster/autovacuum.c

@@ -2911,7 +2911,7 @@ table_recheck_autovac(Oid relid, HTAB *table_toast_map,
 		 */
 		tab->at_dobalance =
 			!(avopts && (avopts->vacuum_cost_limit > 0 ||
-						 avopts->vacuum_cost_delay > 0));
+						 avopts->vacuum_cost_delay >= 0));
 	}
 
 	heap_freetuple(classTup);