3628 [google_sign_in_web] Migrate to null-safety (cla: yes, nnbd, p: google_sign_in, platform-web, waiting for tree to go green)

3631 [file_selector] Migrate to null safety (cla: yes, nnbd, p: file_selector)

3641 [camera] Stable release for null safety. (cla: yes, nnbd, p: camera)

3645 [connectivity] migrate unit tests to null safety (cla: yes, nnbd, p: connectivity)

3646 move integration test to null safety for multiple plugins. (cla: yes, nnbd, p: connectivity, p: device_info, p: image_picker, p: in_app_purchase, p: quick_actions, p: webview_flutter, p: wifi_info_flutter)