[Breaking Change] Make CupertinoDatePicker & CupertinoTimerPicker's background color nullable, and make their default background color null

[Weiyu Huang]

(This change is announced because it affects goldens).

In https://github.com/flutter/flutter/pull/39919 I'm proposing to make the background color of CupertinoDatePicker & CupertinoTimerPicker nullable, and make the default background color null (which skips background painting), as shown below:

CupertinoDatePicker over a black background, with backgroundColor = null, and text color explicitly set to blue. 

This change is aiming to improve the fidelity of the said cupertino widgets, as their native counterpart has a transparent background. 

If you have any concerns regarding this change, please leave a comment in https://github.com/flutter/flutter/pull/39919, or email me directly. Thank you!