[#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.
I think an addition here might be a good regression as well:
```ruby
Resque.enqueued?(SomeUniqueJob).should be false
Resque.enqueue SomeUniqueJob
Resque.enqueued?(SomeUniqueJob).should be true
```