When no args or flags are passed, CLI version of amypet raises an error

[jhuguetn]

Perhaps in such cases the application could just dump the help or usage instead of an error trace. See current behaviour below:

root@d1c0b0de67d6:/# amypet
Traceback (most recent call last):
  File "/usr/local/bin/amypet", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.8/site-packages/amypet/gui.py", line 326, in main
    opts = parser.parse_args(args=args)
  File "/usr/local/lib/python3.8/argparse.py", line 1768, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/local/lib/python3.8/argparse.py", line 1800, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/local/lib/python3.8/argparse.py", line 2035, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, NoneType found