Fix empty body issue when GET fails

[germanop]

If GET fails, the body is empty, so an attempt to convert the body to JSON raises an exception.

The solution here is to return a valid empty JSON body to the caller, that might expect a valid JSON.

This problem has been tackled in other places (but not in set_user_role nor set_grafana_put) in different ways.

• get_grafana_version does not attempt anything if status is not 200
• log_response catches ValueError exceptions and then logs r.text (fine, because print can handle None)
• set_grafana_post catches ValueError and then returns r.text.

I did not go for the same approach as set_grafana_post because I believe the caller should always receive a valid JSON.

[mt3593]

@abalalaev or anyone else, any chance of getting this merged?

[mt3593]

Also #279 appears to fix the same bug linked to issue #277