[BUGFIX] Prevent 503 error when field is NULL

[YKWeyer]

Our editors managed to trigger a 503 error (message: trim() expects parameter 1 to be string, null given) in the backend page module by creating menu_categorized_content objects with selected_categories, then turning them into menu_section objects.

Having any value set in DB for selected_categories is apparently enough to enter StandardContentPreviewRenderer::generateListForMenuContentTypes .
https://github.com/TYPO3/typo3/blob/11.5/typo3/sysext/backend/Classes/Preview/StandardContentPreviewRenderer.php#L198 even if the CType doesn't use it.

A possible fix could be to check the right field for the right CType. But a perhaps more robust and flexible solution would be to ensure NULL values would be turned into empty strings.

How to reproduce

Reproduced with TYPO3 version 11.5.34

1. On a new page, create a menu_categorized_content ContentElement, and select one category in selected_categories
2. Save
3. Update the CType to be menu_section
4. Save and close
5. Opening the page module for it will result in the following exception:

Whoops, looks like something went wrong.
(1/1) TypeError

trim() expects parameter 1 to be string, null given

[TYPO3IncTeam]

Thank you for your contribution to TYPO3. We are using Gerrit Code Review for our contributions and took the liberty to convert your pull request to a review in our review system.
You can find your patch at: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82793
For further information on how to contribute have a look at https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/