auth: Remove unnecessary remote_user=None code path.

This code path was only required because we had remote_user set as a
positional argument here, and thus we'd be running this auth backend's
code when actually using another auth backend (due to how Django auth
backends are selected based on argument signature).

zerver/tests/test_auth_backends.py

@@ -1626,11 +1626,6 @@ class TestZulipRemoteUserBackend(ZulipTestCase):
             self.assertEqual(result.status_code, 302)
             self.assertEqual(get_session_dict_user(self.client.session), user_profile.id)
 
-    def test_authenticate_with_missing_user(self):
-        # type: () -> None
-        backend = ZulipRemoteUserBackend()
-        self.assertIs(backend.authenticate(None), None)
-
     def test_login_success_with_sso_append_domain(self):
         # type: () -> None
         username = 'hamlet'

zproject/backends.py

@@ -412,10 +412,9 @@ class ZulipRemoteUserBackend(RemoteUserBackend):
 
     def authenticate(self, remote_user: Optional[str],
                      realm: Optional[Realm]=None) -> Optional[UserProfile]:
+        assert remote_user is not None
         if realm is None:
             return None
-        if remote_user is None:
-            return None
 
         email = remote_user_to_email(remote_user)
         user_profile = common_get_active_user_by_email(email)