fix(messaging,nnbd): regression in RemoteMessage.fromMap() causing silent failure

packages/firebase_messaging/firebase_messaging_platform_interface/lib/src/remote_message.dart

@@ -28,15 +28,15 @@ class RemoteMessage {
       senderId: map['senderId'],
       category: map['category'],
       collapseKey: map['collapseKey'],
-      contentAvailable: map['contentAvailable'],
+      contentAvailable: map['contentAvailable'] ?? false,
       data: map['data'] == null
           ? <String, dynamic>{}
           : Map<String, dynamic>.from(map['data']),
       from: map['from'],
       // Note: using toString on messageId as it can be an int or string when being sent from native.
       messageId: map['messageId']?.toString(),
       messageType: map['messageType'],
-      mutableContent: map['mutableContent'],
+      mutableContent: map['mutableContent'] ?? false,
       notification: map['notification'] == null
           ? null
           : RemoteNotification.fromMap(

packages/firebase_messaging/firebase_messaging_platform_interface/test/remote_message_test.dart

@@ -0,0 +1,47 @@
+import 'package:firebase_messaging_platform_interface/firebase_messaging_platform_interface.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Map<String, dynamic>? mockMessageMap;
+
+  group('RemoteMessage: handle optional fields under sound null safety:', () {
+    setUp(() {
+      // basic message received, optional fields missing
+      mockMessageMap = {
+        'token': '012345678...',
+        'data': {
+          'via': 'FlutterFire Cloud Messaging!!!',
+          'count': 1,
+        },
+        'notification': {
+          'title': 'Hello FlutterFire!',
+          'body': 'This notification was created from unit tests!',
+        },
+      };
+    });
+
+    test('Optional fields should be set false if absent', () {
+      final message = RemoteMessage.fromMap(mockMessageMap!);
+
+      expect(message.data, mockMessageMap!['data']);
+      expect(message.notification, isA<RemoteNotification>());
+
+      expect(message.mutableContent, false);
+      expect(message.contentAvailable, false);
+    });
+
+    test('Optional fields should be set to given value if present', () {
+      // add values for optional fields
+      mockMessageMap!['mutableContent'] = true;
+      mockMessageMap!['contentAvailable'] = true;
+
+      final message = RemoteMessage.fromMap(mockMessageMap!);
+
+      expect(message.data, mockMessageMap!['data']);
+      expect(message.notification, isA<RemoteNotification>());
+
+      expect(message.mutableContent, true);
+      expect(message.contentAvailable, true);
+    });
+  });
+}