Readding the changes from 3260646 to avoid array_flip() message.

Closes #3432321

src/EventSubscriber/MigrationImportSync.php

@@ -87,7 +87,7 @@ class MigrationImportSync implements EventSubscriberInterface {
 
         if (!in_array($map_source_id, $source_id_values, TRUE)) {
           $destination_ids = $id_map->currentDestination();
-          if ($destination_ids !== NULL) {
+          if ($destination_ids !== NULL && $destination_ids !== []) {
             $this->dispatchRowDeleteEvent(MigrateEvents::PRE_ROW_DELETE, $migration, $destination_ids);
             if (class_exists(MigratePlusEvents::class)) {
               $this->dispatchRowDeleteEvent(MigratePlusEvents::MISSING_SOURCE_ITEM, $migration, $destination_ids);