Make PageView.controller nullable

If a controller isn't provided in the constructor, the controller member is null. This makes PageView and its controller property consistent with other widgets.

Before:

pageView.controller.page

After:

pageView.controller!.page

Landed in version: 3.19.0-12.0.pre
In stable release: Not yet

Relevant issues:

• PageView uses global controller, that is never disposed. (Issue 141119)