Fix Button Regression with null image source

[tj-devel709]

This PR fixes an null reference issue that occurs when an button has its image set by UIButton.SetImage. Instead of using the Button.ImageSource.ToString() to see if the image has changed without needing to hold a reference to the image, the code now uses the UIButton.CurrentImage hash to determine if the image has changed. This helps us know if the image was changed in those situations where the Button.ImageSource is null but the UIButton.CurrentImage is still set.

Description of Change

Issues Fixed

Fixes #25409