Return global config only when /config key exists in DCS (patroni#3393)

Close patroni#3389

Author: CyberDem0n

patroni/api.py

@@ -567,7 +567,7 @@ def do_GET_config(self) -> None:
         ``502`` instead.
         """
         cluster = self.server.patroni.dcs.cluster or self.server.patroni.dcs.get_cluster()
-        if cluster.config:
+        if cluster.config and cluster.config.modify_version:
             self._write_json_response(200, cluster.config.data)
         else:
             self.send_error(502)