Fix vacuum_cost_delay check for balance calculation.

 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)