[BUGFIX] Avoid exception when no metadata given

[franzkugelmann]

When Tika cannot extract metadata for a single file, the whole process l(ike in scheduler run) dies with exception Core: Exception handler (CLI): Uncaught TYPO3 Exception: Argument 2 passed to Causal\Extractor\Resource\Event\AfterMetadataExtractedEvent::__construct() must be of the type array, null given