Issue #3592157: Add explicit nullable getPaneData key

Closes #3592157

getPaneData() accepted NULL through the default key argument, which triggers the PHP 8.4 implicit nullable parameter deprecation.

Changed the signature to use an explicit nullable string and updated the docblock. No behavior change.

Checked locally:

• php -l src/Entity/PagererPreset.php
• • composer validate --no-check-publish
• • git diff --check

composer validate only reports the existing missing license warning.

Credit to savage1974 for reporting.