[bug] GraphQL server returns 500 when sending empty json payload

[nitram509]

Describe the bug

When empty or broken JSON is send to the /query endpoint, a 500 internal error is returned, which is mis-leading and should be a 400 bad request instead.

To Reproduce
Steps to reproduce the behavior:

1. Start the graphql server
2. fire a post request to the /query endpoint, wih a payload of 0 bytes

Actual behaviour

Input (raw) sent:

POST /query HTTP/1.1
user-agent: guac-visualizer
accept: application/json
accept-encoding: gzip, deflate, br
Host: localhost:8080
Connection: keep-alive
Content-Length: 0

The server returns status 500 unexpected end of JSON input
Raw reponse from the server:

HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Sun, 13 Oct 2024 18:06:52 GMT
Content-Length: 29

unexpected end of JSON input

Expected behaviour
Since this is related to user input data, a status code 400 "bad request" should be used.
Also the error message could be a bit more proactive: e.g. fmt.Sprintf("provided input json couldn't be parsed. likely it was empty or was wrongly structured; error message: %v", err.String()).
When I developed a reverse proxy for the visualizer, I was completely puzzled and mis-lead because of the 500 error and thought of a bigger issue inside GraphQL, looked in to the error logs, but couldn't find. spend too much time thinking, until realizing my error was "just" in the empty json payload.

Screenshots
n/a

GUAC version
build from repository, as of Oct. 14th, 2024

Additional context
n/a