csvColumns can be 0, so the check must be against an empty string

Change-Id: I42e23b4bb4e5a69b8260f82a68fd21ab7dda4c0f
Resolves: #54911
Reviewed-on: https://review.typo3.org/26900
Reviewed-by: Oliver Klee
Reviewed-by: Thomas Skierlo
Tested-by: Thomas Skierlo
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau