`checkboxInputType` and `ngList` directives need to have special logic for whether
they are empty or not. Previously this had been hard coded into their
own directives or the `ngRequired` directive. This made it difficult to handle
these special cases.
This change factors out the question of whether an input is empty into a method
`$isEmpty` on the `ngModelController`. The `ngRequired` directive now uses this
method when testing for validity and directives, such as `checkbox` or `ngList`
can override it to apply logic specific to their needs.
Closes#3490, #3658, #2594
b56b21a898