[#83330610] Handle nil job args; make tests Rspec 3.x compatible

For some reason we still don't know, in QA1, empty job args are being passed as nil instead of [ ]. I think the gem code should gracefully handle this case.
Thanks for looking, Ryan. I did that at first, actually, but it passes anyway, because I think the Resque code that's involved must be converting it. Even though somehow in QA1 it's not doing that. Could be a difference in Resque versions, but I think this change is good anyway.