TYPO3 Core

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.

1. 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

This issue was automatically created from https://github.com/TYPO3/typo3/pull/478