Add possibility of return Empty

[pauloabreu]

Resume

Sometimes, if has not contact in a period of time, it can cause an error.

Solution

I created a new field in protobuf called "is_valid", so if the response is not ok, it will return a empty object with is_valid = False

System check identified no issues (0 silenced).
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Ran 4 tests in 1.380s
OK

reference: Ilhasoft/weni-protobuffers#78