Fix the character field of RawKeyEvent is always null on iOS (#135100)

Fixes https://github.com/flutter/flutter/issues/133956

packages/flutter/lib/src/services/raw_keyboard.dart

@@ -356,6 +356,10 @@ abstract class RawKeyEvent with Diagnosticable {
             keyCode: message['keyCode'] as int? ?? 0,
             modifiers: message['modifiers'] as int? ?? 0,
           );
+          final Object? characters = message['characters'];
+          if (characters is String && characters.isNotEmpty) {
+            character = characters;
+          }
         case 'linux':
           final int unicodeScalarValues = message['unicodeScalarValues'] as int? ?? 0;
           data = RawKeyEventDataLinux(

packages/flutter/test/services/raw_keyboard_test.dart

@@ -19,7 +19,7 @@ class _ModifierCheck {
 void main() {
   group('RawKeyboard', () {
     testWidgetsWithLeakTracking('The correct character is produced', (WidgetTester tester) async {
-      for (final String platform in <String>['linux', 'android', 'macos', 'fuchsia', 'windows']) {
+      for (final String platform in <String>['linux', 'android', 'macos', 'fuchsia', 'windows', 'ios']) {
         String character = '';
         void handleKey(RawKeyEvent event) {
           expect(event.character, equals(character), reason: 'on $platform');
@@ -34,7 +34,7 @@ void main() {
     });
 
     testWidgetsWithLeakTracking('No character is produced for non-printables', (WidgetTester tester) async {
-      for (final String platform in <String>['linux', 'android', 'macos', 'fuchsia', 'windows', 'web']) {
+      for (final String platform in <String>['linux', 'android', 'macos', 'fuchsia', 'windows', 'web', 'ios']) {
         void handleKey(RawKeyEvent event) {
           expect(event.character, isNull, reason: 'on $platform');
         }