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

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`.

lib/galaxy/jobs/handler.py

@@ -1216,12 +1216,16 @@ def get_job_runner(self, job_wrapper, get_task_runner=False):
         except KeyError:
             log.error(f"({job_wrapper.job_id}) Invalid job runner: {runner_name}")
             job_wrapper.fail(DEFAULT_JOB_RUNNER_FAILURE_MESSAGE)
+            return None
         if get_task_runner and job_wrapper.can_split() and runner.runner_name != "PulsarJobRunner":
             return self.job_runners["tasks"]
         return runner
 
     def put(self, job_wrapper):
         runner = self.get_job_runner(job_wrapper, get_task_runner=True)
+        if runner is None:
+            # Something went wrong, we've already failed the job wrapper
+            return
         if isinstance(job_wrapper, TaskWrapper):
             # DBTODO Refactor
             log.debug(f"({job_wrapper.job_id}) Dispatching task {job_wrapper.task_id} to task runner")