Empty request with json content type fails with a JSONDecodeError when using the history panel

[MDziwny]

If an API request is made with an empty body and the content type application/json, the history panel raises an error and the whole API request fails.

For instance:

GET /api/whatever
Content-Type: application/json

Body: 

Result in 500 Internal Server Error, JSONDecodeError, Expecting value: line 1 column 1 (char 0)

We could argue that an empty body is not a valid application/json body (https://greenbytes.de/tech/webdav/rfc7159.html#values), but in this case, I believe it's not the role of django debug toolbar to raise an error if django accepts this kind of requests.