[BUGFIX] Followup: Mandatory for Selectbox with TCA not possible

Followup to: https://review.typo3.org/28625
No use-statement for MathUtility in 6.1 yet.

Change-Id: Iffa9c7cd9ab41831e34c40d7b3ada530ba73ac9c
Resolves: #24925
Resolves: #24871
Releases: 6.1
Reviewed-on: https://review.typo3.org/29507
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny