[BUGFIX] New content elements are always stored on pid 0

Due to patch https://review.typo3.org/#/c/30305/ the string comparison
on colPos fails and new content elements are always stored on pid 0.
This patch corrects the check for an integer colPos type by setting the
unused variable to NULL.

Resolves: #59059
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: Iecd7f0cacf5c9315d882eebeb3893bcfa63ae7eb
Reviewed-on: https://review.typo3.org/30416
Reviewed-by: Markus Klein
Tested-by: Markus Klein