Prevent Panic if PG Deployments Scaled Down

If the primary and all replica Deployments are scaled to zero outside
of the context of the PostgreSQL Operator (e.g. by using
'kubectl scale deployment --replicas=0' instead of
'pgo update cluster --shutdown') and the associated pgcluster for the
Deployment(s) being scaled is not manually set to the proper "shutdown"
status, then the PostgreSQL Operator could panic as it continues to
attempt to synchronize Patroni/PostgreSQL configuration for the PG
cluster (specifically because no pods are found for the cluster).  This
commit protects against a panic in this scenario by simply logging an
error when no Pods are found during an attempt to sync configuration.

Issue: [ch9278]

Author: andrewlecuyer

operator/config/localdb.go

@@ -212,10 +212,10 @@ func (l *LocalDB) Update(configName string, localDBConfig LocalDBConfig) error {
 	return nil
 }
 
-// apply applies the configuration stored in the CusterConig's configMap for a
-// specific database server to that server.  This is done by updating the contents of that
-// database server's local configuration with the configuration for that cluster stored in
-// the LocalDB's configMap, and the issuing a Patroni "reload" for that specific server.
+// apply applies the configuration stored in the cluster ConfigMap for a specific database server
+// to that server.  This is done by updating the contents of that database server's local
+// configuration with the configuration for that cluster stored in the LocalDB's configMap, and
+// then issuing a Patroni "reload" for that specific server.
 func (l *LocalDB) apply(configName string) error {
 
 	clusterName := l.configMap.GetObjectMeta().GetLabels()[config.LABEL_PG_CLUSTER]
@@ -238,6 +238,11 @@ func (l *LocalDB) apply(configName string) error {
 	if err != nil {
 		return err
 	}
+	// if the pod list is empty, also return an error
+	if len(dbPodList.Items) == 0 {
+		return fmt.Errorf("no pod found for %q", clusterName)
+	}
+
 	dbPod := &dbPodList.Items[0]
 
 	// add the config name and patroni port as params for the call to the apply & reload script