[BUGFIX] Allow backend layout without any colPos

Resolves: #97268
Releases: 11.5
Change-Id: I389cb4d076f1595347c1b4c28df8ec84c373c552
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74124
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>

typo3/sysext/backend/Classes/Controller/PageLayoutController.php

@@ -90,7 +90,7 @@ class PageLayoutController
      *
      * @var string
      */
-    protected $colPosList;
+    protected $colPosList = '';
 
     /**
      * Currently selected language for editing content elements