Update README and version

Author: hienlh

.github/workflows/test.yml

@@ -4,6 +4,8 @@
 
 ### OneSignal Flutter SDK [![Build Status](https://travis-ci.org/OneSignal/OneSignal-Flutter-SDK.svg?branch=master)](https://travis-ci.org/OneSignal/OneSignal-Flutter-SDK)
 
+> Now supports null safety.
+
 ---
 
 [OneSignal](https://onesignal.com/) is a free push notification service for mobile apps. This SDK makes it easy to integrate your Flutter iOS and/or Android apps with OneSignal.

README.md

@@ -1,6 +1,6 @@
 name: onesignal_flutter
 description: OneSignal is a free push notification service for mobile apps. This plugin makes it easy to integrate your flutter app with OneSignal
-version: 2.6.3
+version: 2.7.0-nullsafety
 author: Brad Hesse <[email protected]>, Josh Kasten <[email protected]>
 homepage: https://github.com/OneSignal/OneSignal-Flutter-SDK