No _data in response in some cases

[denis-ignatenko]

When calling Connection authenticate there is not data in response. In interactive console exception raised
`--> 236 for tpl in self._data:
237 output.extend(("- ", repr(tpl), "\n"))
238 if len(output) > 0:

TypeError: 'NoneType' object is not iterable`

tarantool-python/tarantool/response.py

Line 236 in debe676

for tpl in self._data:

Possible fix:
for i in r._data or ()