🩹(backend) enable resource server authentication if properly configured

Tests are missing, let's ship it, I'll open an issue.

Without such protection, the whole app would crash if the resource server is
not configured. The fallback backend would return an appropriate error to
the client if the resource server is improperly configured.

src/backend/core/resource_server/clients.py

@@ -1,5 +1,7 @@
 """Resource Server Clients classes"""
 
+from django.core.exceptions import ImproperlyConfigured
+
 import requests
 from joserfc.jwk import KeySet
 
@@ -28,6 +30,11 @@ class AuthorizationServerClient:
         timeout,
         proxy,
     ):
+        if not url or not url_jwks or not url_introspection:
+            raise ImproperlyConfigured(
+                "Could not instantiate AuthorizationServerClient, some parameters are missing."
+            )
+
         self.url = url
         self._url_introspection = url_introspection
         self._url_jwks = url_jwks