fix for step with gres count of zero

Fix for possible error if job has GRES, but the step explicitly requests a
GRES count of zero.
srun --gres=non to work without alloc

Make "srun --gres=none ..." work when executed without a job allocation
(i.e. srun creates the allocation plus the step). Previous logic would
try to create the job with a gres value of "none".