Handle nil job args; make be_true/be_false specs pass under RSpec 3.x

To the author:
The reason for this PR is that in our test environment (CentOS), we're somehow getting nil for the job args but an empty Array in local (OS X) testing. We're using resque-scheduler v2.0.0, resque 1.25.2, and resque-loner 1.2.1.

Do you have any idea why/how this could be happening?

We figure this change will make the code more robust in any case, but we'd still like to understand what is happening. Without this change, we have to explicitly pass a dummy arg to our no-arg jobs, which obviously isn't very nice.

Thanks!