`$isEmpty` fixes

We should only use $isEmpty on $viewValue not for validating models.
Again this is part of the revert of Tobias's commit.
It doesn't actually have anything to do with `$isEmpty`.

I am not sure of the reasoning behind checking both values. It would seem to me that we should be checking the `$viewValue` only as you say, since this is what the input[url] directive has control over. You can't assume anything about the model, since users can add in their own crazy parsers via their own directives if they want.