Fix empty $messageBlock on CheckboxFields

From 2.4.13 the Message attribute is set to Text, so every checkbox now show an empty $messageBlock,
See https://github.com/silverstripe/silverstripe-framework/issues/2489 or http://www.silverstripe.org/general-questions/show/25254
Merge pull request #2716 from gbonanome/patch-1

Fix empty $messageBlock on CheckboxField