Set View#subviews to empty list instead of null by default.

This slightly reverts change from 3140f8e5f231dce2b1f90161cfdec10f85909a9e, but keeps its performance optimisation.

It also fixes reasoning about type of View#subviews. It is always list now.
Update view.coffee
Merge pull request #561 from chaplinjs/topics/subviews-consistency

Set View#subviews to empty list instead of null by default.