Change ISA worker, job comparision logic.

Description

I believe the current logic is flawed in that if job_isas is not None and worker_isas is a [], then an error occurs when doing bitwise & of a list and set.

Also, returning empty sets when the keys aren't found makes sense since even though the worker.proto allows for duplicate ISA keys, worker_capabilites and job_requirements are both dictionaries of sets, therefore we lose the duplicates when adding to either.