[BUGFIX] Mitigate a TypeError in `StandardContentPreviewRenderer`

An editor can trigger a TypeError exception leading to 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` and then turning them into
`menu_section` objects.

This change mitigates the type error by using a null-coalesce
fallback passing an invalid type to the `trim()` method in
`StandardContentPreviewRenderer::generateListForMenuContentTypes()`.

Resolves: #103052
Releases: main, 12.4, 11.5
Change-Id: I4c0ecbd20c7cdfee22fa324f6b80391be47a1b88
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82845


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>

typo3/sysext/backend/Classes/Preview/StandardContentPreviewRenderer.php

@@ -335,7 +335,7 @@ class StandardContentPreviewRenderer implements PreviewRendererInterface, Logger
             $table = 'sys_category';
             $field = 'selected_categories';
         }
-        if (trim($record[$field]) === '') {
+        if (trim($record[$field] ?? '') === '') {
             return '';
         }
         $content = '';