Prefer void to null (#22977)

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

examples/platform_channel_swift/lib/main.dart

diff --git a/examples/platform_channel_swift/lib/main.dart b/examples/platform_channel_swift/lib/main.dart
index 8739a01..48eddc8 100644
--- a/examples/platform_channel_swift/lib/main.dart
+++ b/examples/platform_channel_swift/lib/main.dart
@@ -21,7 +21,7 @@
   String _batteryLevel = 'Battery level: unknown.';
   String _chargingStatus = 'Battery status: unknown.';
 
-  Future<Null> _getBatteryLevel() async {
+  Future<void> _getBatteryLevel() async {
     String batteryLevel;
     try {
       final int result = await methodChannel.invokeMethod('getBatteryLevel');