Setters, attach()/detach() is never called on components with null expression attributes

[taisph]

Setters, attach()/detach() are never called on a component with an attribute that evaluates to null or an empty string, ie:

<mycomp id="{{null}}"></mycomp>
<mycomp id="{{''}}"></mycomp>

Valueless or empty string attributes (non-expression/mustached) does however trigger the methods.

Link to demonstration:
https://github.com/taisph/angular.dart-attach-test