fix unit fixed setting postgresql version into app_peer_data

src/charm.py

@@ -1586,7 +1586,7 @@ def _validate_database_version(self):
 
         if self.unit.is_leader() and peer_db_version is None:
             _psql_version = self._patroni.get_postgresql_version()
-            if _psql_version is not None:
+            if not _psql_version:
                 self.app_peer_data.update({"database-version": _psql_version})
             return