[BUGFIX] Mandatory for Selectbox with TCA not possible

It's not possible to create a backend selectbox
that is mandatory with only 1 item selectable.

Releases: 6.2, 6.1
Resolves: #24925
Resolves: #24871
Change-Id: Idaef6475f61c70cd8c3f6074ccd0b0195d90c581
Reviewed-on: https://review.typo3.org/28625
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind