greader.php: only return 'OK' on greader.php without query parameters

[Frenzie]

It'd be clearer if the API endpoint didn't return OK when adding the Email and Passwd parameters. Whether it should instead return OK if they're correct is debatable.

In reply to #5856 (comment).

How to test the feature manually:

Before:

$ http 'http://localhost:8080/api/greader.php?Email=<user>&Passwd=<pass>'
HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Length: 2
Content-Type: text/html; charset=UTF-8
Date: Tue, 26 Mar 2024 20:24:45 GMT
Keep-Alive: timeout=5, max=100
Server: Apache/2.4.54 (Unix)
X-Powered-By: PHP/8.0.26

OK

After:

$ http 'http://localhost:8080/api/greader.php?Email=<user>&Passwd=<pass>'
HTTP/1.1 400 Bad Request
Connection: close
Content-Length: 12
Content-Type: text/plain; charset=UTF-8
Date: Tue, 26 Mar 2024 20:25:14 GMT
Server: Apache/2.4.54 (Unix)
X-Powered-By: PHP/8.0.26

Bad Request!

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated