[FIX] core: jsonrpc sends 400 for non json request

Send a request to any json-rpc route with a HTTP header
`Content-Type: application/json-rpc` but send non-json or
non-jsonrpc data in the body. The application crashes with
a 500 Internal Server Error instead of a 400 Bad Request one

closes odoo/odoo#122437

Closes: #122048
Signed-off-by: Julien Castiaux (juc) <juc@odoo.com>