[BUGFIX] Avoid PHP Warning if there are empty columns in Page Module

Change-Id: I218065a3445211d88ce131ecf23da3a947eb2e70
Resolves: #44540
Branches: 6.1, 6.0
Reviewed-on: https://review.typo3.org/17562
Reviewed-by: Mattias Nilsson
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer