3644 [path_provider] Migrate all integration tests to NNBD (cla: yes, p: path_provider, platform-linux, platform-macos, platform-windows)

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)

3647 Update pull_request_label.yml to make actions pointing to refs (cla: yes)