ValueError: min() arg is an empty sequence

[yjwong]

This looks similar to #23 and #29 but in a different location.

Python version 3.9.2
Redis version 6.0.8

/ # rma -b ram -s redis-master.production                   [12/57307]
Match *: 100%|██████████████▉| 55430/55447 [00:01<00:00, 34761.63it/s]
Aggregating keys by pattern and type                                                                                                        Apply rules
Processing Set patterns:   0%|                  | 0/6 [00:00<?, ?it/s]
Traceback (most recent call last):
  File "/usr/local/bin/rma", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.9/site-packages/rma/cli/rma_cli.py", line 105, in main
    app.run()
  File "/usr/local/lib/python3.9/site-packages/rma/application.py", line 143, in run
    str_res.append(self.do_ram(keys))
  File "/usr/local/lib/python3.9/site-packages/rma/application.py", line 176, in do_ram
    ret[redis_type] = rule.analyze(keys=aggregate_patterns, total=total_keys)
  File "/usr/local/lib/python3.9/site-packages/rma/rule/Set.py", line 90, in analyze
    agg = SetAggregator(progress_iterator((SetStatEntry(x, self.redis) for x in data), progress), len(data))
  File "/usr/local/lib/python3.9/site-packages/rma/rule/Set.py", line 59, in __init__
    self.ttlMin = min(ttls)
ValueError: min() arg is an empty sequence