ReFrame crashes when using the `squeue` scheduler backend

[vkarak]

Using this config;

                {
                    'name': 'c5n',
                    'scheduler': 'squeue',
                    'launcher': 'srun',
                    'access': ['-p c5n'],
                    'environs': ['builtin'],
                }

Running the generic hellocheck we get the following error:

./bin/reframe: run session stopped: attribute error: 'NoneType' object has no attribute 'submit_time'
./bin/reframe: Traceback (most recent call last):
  File "/home/ec2-user/reframe/reframe/frontend/cli.py", line 968, in main
    runner.runall(testcases, restored_cases)
  File "/home/ec2-user/reframe/reframe/frontend/executors/__init__.py", line 431, in runall
    self._runall(testcases)
  File "/home/ec2-user/reframe/reframe/frontend/executors/__init__.py", line 504, in _runall
    self._policy.exit()
  File "/home/ec2-user/reframe/reframe/frontend/executors/policies.py", line 532, in exit
    self._poll_tasks()
  File "/home/ec2-user/reframe/reframe/frontend/executors/policies.py", line 446, in _poll_tasks
    part.scheduler.poll(*part_jobs)
  File "/home/ec2-user/reframe/reframe/core/schedulers/slurm.py", line 557, in poll
    m = max(job.submit_time for job in jobs)
  File "/home/ec2-user/reframe/reframe/core/schedulers/slurm.py", line 557, in <genexpr>
    m = max(job.submit_time for job in jobs)
AttributeError: 'NoneType' object has no attribute 'submit_time'