fix($parse): create undefined arrays in assigements

Fixes #9820 

surpassingly I can see only very limited number of tests for the `assign`:
https://github.com/angular/angular.js/blob/2a0254e1816b64c1235e3d71af1eefe5466d1fe3/test/ng/parseSpec.js#L1143

But maybe I'm not looking at the right place...