Crash when trying to clean up the cache/out of space

[transplier]

So, I let my disk space run out (plug for #994 :) ). Frigate helpfully tried to clean up /tmp/cache but ended up crashing:

frigate.events                 WARNING : More than 90% of the cache is used.
frigate.events                 WARNING : Consider increasing space available at /tmp/cache or reducing max_seconds in your clips config.
frigate.events                 WARNING : Proactively cleaning up the cache...
Exception in thread event_processor:
Traceback (most recent call last):
  File "/opt/frigate/frigate/events.py", line 193, in run
    event_type, camera, event_data = self.event_queue.get(timeout=10)
  File "/usr/lib/python3.8/multiprocessing/queues.py", line 108, in get
    raise Empty
_queue.Empty

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/opt/frigate/frigate/events.py", line 196, in run
    self.refresh_cache()
  File "/opt/frigate/frigate/events.py", line 121, in refresh_cache
    oldest_clip = min(self.cached_clips.values(), key=lambda x:x['start_time'])
ValueError: min() arg is an empty sequence

Sounds like some better error handling would be helpful here.