fix(ngModel): don’t clear the model when an external validator failed

Calling `ctrl.$setValidity()` with a an error key that 
does not belong to a validator in `ctrl.$validator` should
not result in setting the model to `undefined` on the next
input change. This bug was introduced in 1.3.0-beta.12.

Closes #8357
Fixes #8080
I will add `noop` explicitly, so that it is clear that we do want to do nothing when we have an error there. This makes sense as `validationDone` also calls the `doneCallback` conditionally...