[BUGFIX] Prevent throw on empty source record

[kauz56]

@dmitryd Had a case where translation would get stuck on some records with the following message:

Uncaught TYPO3 Exception: Dmitryd\DdDeepl\Service\DeeplTranslationService::translateRecord(): Argument #2 ($record) must be of type array, null given, called in .../vendor/dmitryd/dd-deepl/Classes/Hook/DataHandlerTranslationHook.php on line 86

Didn't have time to investigate this further, but this seemed to resolve the issue without causing any noticable problem with the translated records. This was a "standard" site using an unmodified dd_deepl v12.7.0. Languages were en -> es. A glossary was used.