✅(backend) handle empty subscription string

Handle case where sub value is an empty string instead of None.
Previously this edge case would cause unexpected behavior.

Related to previous commit that added the test coverage.

src/backend/core/authentication/backends.py

@@ -66,7 +66,7 @@ class OIDCAuthenticationBackend(MozillaOIDCAuthenticationBackend):
         user_info = self.get_userinfo(access_token, id_token, payload)
         sub = user_info.get("sub")
 
-        if sub is None:
+        if not sub:
             raise SuspiciousOperation(
                 _("User info contained no recognizable user identification")
             )