Make ol.Map#setView do not accept undefined

With this change map.setView(null) is permitted, but map.setView(undefined) is not. map.getView now always returns a defined value, which may be null is setView was called with null.
Do not exec animation functions when frameState is null
Rotate control should not act on a null view
Zoom control should not act on a null view
Merge pull request #2722 from elemoine/nullview

Make ol.Map#setView do not accept undefined