Fix undefined array index in DetectSerializedValue Solr hook

[lneugebauer]

New Pull Request checklist

PR Type

What kind of change does this PR introduce?

• [ ] [FEATURE] - A new feature
• [x] [BUGFIX] - A bug fix
• [ ] [TASK] - Any task, which is not a new feature or bugfix
• [ ] [TEST] - Adding missing tests
• [ ] [DOC] - Documentation only changes
• [ ] [WIP] - Work in progress tag, should not be present when creating pull requests

Breaking change

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Description

System

• Apache 2.5.54
• PHP 8.0
• TYPO3 11.5.24
• EXT:Solr 11.5.1
• Solr 11.5.1 -> Docker Hub

Error message

While indexing products the indexer throws the following error message for fields that do not have a user function defined. Like keywords for example.

1476107295: TYPO3\CMS\Core\Error\Exception: PHP Warning: Undefined array key "userFunc" in /path/to/pxa_product_manager/Classes/Hook/Solr/DetectSerializedValue.php line 32 in /path/to/public/typo3/sysext/core/Classes/Error/ErrorHandler.php:137