Remove the omitempty tag for BatchesInQueue field

[RobertLucian]

This is to fix the following error (from this build https://app.circleci.com/pipelines/github/cortexlabs/cortex/9975/workflows/baf607ea-73ae-4066-be0f-da89a5586909/jobs/12944) that can occur for a batch API:

            # assert jobs
            printer("checking the jobs' responses")
            for job_spec in job_specs:
                job_id: str = job_spec["job_id"]
                job_status = requests.get(f"{api_endpoint}?jobID={job_id}").json()["job_status"]
    
                assert (
>                   job_status["batches_in_queue"] == 0
                ), f"there are still batches in queue ({job_status['batches_in_queue']}) for job ID {job_id}"
E               KeyError: 'batches_in_queue'

So if there are no batches in the queue, then batches_in_queue field won't be present in the batch response. And that's what the KeyError: 'batches_in_queue' error is about.

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checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)