Better error message when expecting JSON but get 204 (No content) response

Nobody has claimed this yet.

If we do a GET request that expects a JSON response, like so:

```robot
${response}= RequestsLibrary.GET ${url}
```

then if the server returns 204 (no content) instead of returning JSON, we get this error:
```
Replacing variables from keyword return value failed: Resolving variable '${response.json()}' failed: JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

I think it'd be nice to have a friendly error message for this. Perhaps something like `Expected the response to contain JSON, but the response was 204 (No Content)` 🙂

Start at the RequestsLibrary.GET keyword and the response.json() handling described in the report, then reproduce the case with a GET request returning HTTP 204. Done means that an empty 204 response produces a friendly, specific error instead of the raw JSONDecodeError.

Written by the indexing model from the issue text.