3599 [connectivity_macos] fix version (cla: yes, nnbd, p: connectivity, platform-macos)

3604 [connectivity_macos] fix flutter version constraint (cla: yes, p: connectivity, platform-macos)

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)