Make PageView.controller nullable

Summary

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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.

Migration guide

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Before:

dart
pageView.controller.page

After:

dart
pageView.controller!.page

Timeline

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Landed in version: 3.19.0-12.0.pre
In stable release: 3.22.0

References

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Relevant issues: