[BUGFIX] Enable CheckboxViewHelper binding to ArrayObject and Null

Currently the checkbox view helper relies on a present
form object which is not necessary any longer since 6.1
as the new property mapper is active by default.

Therefore the viewhelper must be adjusted to be able to
be bound to ArrayObject objects or Null which throws an
exception till now.

Releases: 6.2, 6.1
Resolves: #48628
Change-Id: I35b70fb0a09ddd72da1c64ec5c7c5456e9af9471
Reviewed-on: https://review.typo3.org/21184
Tested-by: Adrian Dymorz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring