Crash on empty cassette

[JayBazuzi]

trafficstars

Repro

1. Run a VCR test
2. Delete the contents of the cassette file in an attempt to force it to regenerate (reference #578)
3. Run the test again

Result

  File "C:\Python37\lib\site-packages\vcr\cassette.py", line 100, in __call__
    return type(self)(self.cls, args_getter)._execute_function(function, args, kwargs)
  File "C:\Python37\lib\site-packages\vcr\cassette.py", line 114, in _execute_function
    return self._handle_function(fn=handle_function)
  File "C:\Python37\lib\site-packages\vcr\cassette.py", line 137, in _handle_function
    with self as cassette:
  File "C:\Python37\lib\site-packages\vcr\cassette.py", line 87, in __enter__
    self.__finish = self._patch_generator(self.cls.load(**cassette_kwargs))
  File "C:\Python37\lib\site-packages\vcr\cassette.py", line 163, in load
    new_cassette._load()
  File "C:\Python37\lib\site-packages\vcr\cassette.py", line 336, in _load
    requests, responses = self._persister.load_cassette(self._path, serializer=self._serializer)
  File "C:\Python37\lib\site-packages\vcr\persisters\filesystem.py", line 15, in load_cassette
    cassette = deserialize(cassette_content, serializer)
  File "C:\Python37\lib\site-packages\vcr\serialize.py", line 44, in deserialize
    requests = [Request._from_dict(r["request"]) for r in data["interactions"]]
TypeError: 'NoneType' object is not subscriptable