[23.1] Fix unbound runner variable when there is an error in the job config

[mvdbeek]

Fixes

UnboundLocalError: local variable 'runner' referenced before assignment
  File "galaxy/jobs/handler.py", line 541, in __handle_waiting_jobs
    self.dispatcher.put(self.job_wrappers.pop(job.id))
  File "galaxy/jobs/handler.py", line 1224, in put
    runner = self.get_job_runner(job_wrapper, get_task_runner=True)
  File "galaxy/jobs/handler.py", line 1221, in get_job_runner
    return runner

from https://sentry.galaxyproject.org/share/issue/a0a0049feb53487f9c84ede3784624b0/ which followed (53095435) Invalid job runner: bridges.

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