Additional changes for receive/replay lsn/lag (patroni#3357)

* Properly report unknown receive lsn/lag
* Do not write 0 receive/replay lsn to DCS

Author: hughcapet

patroni/ha.py

@@ -463,7 +463,10 @@ def touch_member(self) -> bool:
                         self.state_handler.timeline_wal_position()
                     data['xlog_location'] = self._last_wal_lsn = wal_position
                     if not timeline:  # running as a standby
-                        data['receive_lsn'], data['replay_lsn'] = receive_lsn, replay_lsn
+                        if replay_lsn:
+                            data['replay_lsn'] = replay_lsn
+                        if receive_lsn:
+                            data['receive_lsn'] = receive_lsn
                         replication_state = self.state_handler.replication_state()
                         if replication_state:
                             data['replication_state'] = replication_state

patroni/utils.py

@@ -979,7 +979,7 @@ def cluster_as_json(cluster: 'Cluster') -> Dict[str, Any]:
                 lsn_type, lag_type = f'{location}lsn', f'{location}lag'
 
                 lsn = getattr(m, lsn_type)
-                if lsn is None:
+                if not lsn:
                     member[lsn_type] = member[lag_type] = 'unknown'
                 elif cluster_lsn >= lsn:
                     member[lag_type] = cluster_lsn - lsn