feat(stream_chat): upgrade to null safe dependencies

packages/stream_chat/lib/src/models/attachment.dart

@@ -35,7 +35,7 @@ class Attachment {
     this.extraData,
     this.file,
     UploadState uploadState,
-  })  : id = id ?? Uuid().v4(),
+  })  : id = id ?? const Uuid().v4(),
         title = title ?? file?.name,
         localUri = file?.path != null ? Uri.parse(file.path) : null {
     this.uploadState = uploadState ??

packages/stream_chat/lib/src/models/message.dart

@@ -73,7 +73,7 @@ class Message {
     this.deletedAt,
     this.status = MessageSendingStatus.sent,
     this.skipPush,
-  })  : id = id ?? Uuid().v4(),
+  })  : id = id ?? const Uuid().v4(),
         pinExpires = pinExpires?.toUtc();
 
   /// Create a new instance from a json