PDF metadata extraction fails when optional "Trapped" field is NULL

[responsemlanger]

We encountered an issue with PDF metadata extraction in the metadata extension.

The error occurs in:

Classes/Index/PdfMetadataExtractor.php

Error message:

Argument 1 passed to Fab\Metadata\Utility\UnicodeUtility::convert()
must be of the type string, null given

The problematic line is:

$value = $this->getUnicodeUtility()->convert($value);

While debugging, we found that some PDFs contain an optional metadata field:

["Trapped"] => NULL

The extension iterates over all PDF properties and passes values directly into convert(), but null values are not handled.

Suggested fix:

$value = $this->getUnicodeUtility()->convert($value ?? '');

This seems to affect PDFs where optional metadata fields are unset/null, which is valid according to the PDF specification.

Environment:

TYPO3 9.5.5
PHP 7.4