Issue #3514604: Fix for translation merging

Closes #3514604

src/ExternalEntityStorage.php

@@ -725,7 +725,7 @@ class ExternalEntityStorage extends ContentEntityStorageBase implements External
 
       // Check if there are translation data available.
       foreach ($all_raw_data[$id] as $key => $raw_translation_data) {
-        if (str_starts_with($key, self::EXTERNAL_ENTITY_TRANSLATION_SUB_FIELD_PREFIX)) {
+        if (isset($raw_translation_data[$id]) && str_starts_with($key, self::EXTERNAL_ENTITY_TRANSLATION_SUB_FIELD_PREFIX)) {
           try {
             $langcode = str_replace(self::EXTERNAL_ENTITY_TRANSLATION_SUB_FIELD_PREFIX, '', $key);
             $entity_translation_data = $this->extractEntityValuesFromRawData($raw_translation_data[$id]);