fix(ngModel): always format the viewValue as a string for text, url and email types

NgModel will format all scope-based values to string when setting the viewValue for
the associated input element. The formatting, however, only applies to input elements
that contain a text, email, url or blank input type. In the event of a null or undefined
scope or model value, the viewValue will be set to null or undefined instead of being
converted to an empty string.