feat(stream_chat): upgrade to null safe dependencies

packages/stream_chat/test/src/models/attachment_test.dart

@@ -1,12 +1,12 @@
+import 'dart:convert';
 import 'package:stream_chat/src/models/attachment.dart';
 import 'package:stream_chat/src/models/action.dart';
-import 'dart:convert';
 
 import 'package:test/test.dart';
 
 void main() {
   group('src/models/attachment', () {
-    const jsonExample = r'''{
+    const jsonExample = '''{
   "type": "giphy",
   "title": "awesome",
   "title_link": "https://giphy.com/gifs/nrkp3-dance-happy-3o7TKnCdBx5cMg0qti",
@@ -38,22 +38,27 @@ void main() {
 
     test('should parse json correctly', () {
       final attachment = Attachment.fromJson(json.decode(jsonExample));
-      expect(attachment.type, "giphy");
-      expect(attachment.title, "awesome");
-      expect(attachment.titleLink,
-          "https://giphy.com/gifs/nrkp3-dance-happy-3o7TKnCdBx5cMg0qti");
-      expect(attachment.thumbUrl,
-          "https://media0.giphy.com/media/3o7TKnCdBx5cMg0qti/giphy.gif");
+      expect(attachment.type, 'giphy');
+      expect(attachment.title, 'awesome');
+      expect(
+        attachment.titleLink,
+        'https://giphy.com/gifs/nrkp3-dance-happy-3o7TKnCdBx5cMg0qti',
+      );
+      expect(
+        attachment.thumbUrl,
+        'https://media0.giphy.com/media/3o7TKnCdBx5cMg0qti/giphy.gif',
+      );
       expect(attachment.actions, hasLength(3));
       expect(attachment.actions[0], isA<Action>());
     });
 
     test('should serialize to json correctly', () {
       final channel = Attachment(
-          type: "image",
-          title: "soo",
-          titleLink:
-              "https://giphy.com/gifs/nrkp3-dance-happy-3o7TKnCdBx5cMg0qti");
+        type: 'image',
+        title: 'soo',
+        titleLink:
+            'https://giphy.com/gifs/nrkp3-dance-happy-3o7TKnCdBx5cMg0qti',
+      );
 
       expect(
         channel.toJson(),