[BUGFIX] Avoid PHP fatal error if colPos is not string

The newly added class \TYPO3\CMS\Backend\View\PageLayoutViewDrawEmptyColposContent
incorrectly assumes that colPos values are always strings
when they may very well be integers. This patch casts
the possibly-integer value to string before use of trim().

Releases: master
Resolves: #90211
Change-Id: I8fb145c76f2bd6b0bca187be3b4c5b05fd3c3d80
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63044


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>