Fix job generation when using batch_size

When ssbench-worker gets a None type of job from
ssbench-master, it returns a result which is different
from expected results dictionary.
In that case, reporter will fails generating a report.

Currently, ssbench-master might push None type job,
when using batch_size and many unable jobs in the scenario.
(e.g. DELETE).

This patch makes ssbench-master not to generate None
type job and fixes the bug.

Note:
If worker returns unexpected results by another reason,
reporter fails as same as current behavior.