fix($observe): check if the attribute is undefined

Check if the attribute is undefined before manually applying the function because if not an undefined
property is added to the scope of the form controller when the input control does not have a name.

Closes #9707

[#ngEurope](http://ngeurope.org/) /cc @rodyhaddad @vojtajina 
I think what we should do though, is spy on the $$addControl and $$renameControl methods of the form though... I think we'd have to rewrite it to not use `compileInput` to make that work