[BUGFIX] Check minitems for TCAtree

The TCAtree ignores currently the minitems.
This is however e.g. needed to require input for a relation field.

Change-Id: Id972b0e065adf7e0702a8d8f85c469c7fe9ac827
Resolves: #25003
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/7662
Reviewed-by: Philipp Gampe
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter