Tapestry doesn't support validations on checkboxes out of the box (after all, it's either on or off). Perhaps supporting the required validator would not be so bad.
However, it is reasonable to add a validator on the client side, for example, to implement "if this is selected, this must be checked as well" scenarios.
Anyway, because $F() of a checkbox return nulls, this line fails
if (!t.validationError && ! value.blank())
because value is null
This is in Tapestry.FieldEventManager.validateInput