return empty JSON object instead of empty string

`json.loads('')` fails, because the empty string is not valid JSON. I return an empty object from `get_json` instead

I don't know whether it's better to fix this here, or at https://github.com/python-diamond/Diamond/blob/8fa02fa4616c6ebedceaa64f8f7fe24d94d867ba/src/collectors/postfix/postfix.py#L89