hiding value for boolean/null comparators

[mrname]

A little bit of a hack here, but does successfully hide the "value" input element when istrue, isfalse, and isnull operators are chosen. Since it technically populates the value of the element and then hides it, there will be a small annoyance if you switch it back to a field that requires input (you would see the value we stuck in there, i.e. - True/False). However, this seems less annoying than having to type "null" in the box when creating these fields.

[coveralls]

Coverage remained the same at 96.06% when pulling 4c0f246fc438900be43198cf19b30498e7877397 on mrname:issue_8_no_value_for_bool_and_null into b243e0688e3d9f2fd09a4d70b9237611aaac5df5 on modlinltd:develop.