Fixes exception if response.body is "null"

If you do a get() on a path that doesn't exist, the response.body = "null", which raises a JSON::ParserError:

```
irb(main):021:0> r.body
JSON::ParserError: 757: unexpected token at 'null'
```

This patch fixes this by check if the response.body == 'null'. If so, it returns nil, otherwise it re-raises the JSON::ParserError.

```
irb(main):002:0> r.body
=> nil
```