msglist [nfc]: Give MessageItem the SizedBox for trailing whitespace

Instead of having its caller make the SizedBox.

This way, the whitespace will be naturally included when we give the
whole message list's white background color to the individual
MessageItems, coming next.

lib/widgets/message_list.dart

@@ -330,7 +330,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
           case MessageListMessageItem():
             return MessageItem(
               key: ValueKey(data.message.id),
-              trailing: i == 1 ? const SizedBox(height: 8) : const SizedBox(height: 11),
+              trailingWhitespace: i == 1 ? 8 : 11,
               item: data);
         }
       });
@@ -450,11 +450,11 @@ class MessageItem extends StatelessWidget {
   const MessageItem({
     super.key,
     required this.item,
-    this.trailing,
+    this.trailingWhitespace,
   });
 
   final MessageListMessageItem item;
-  final Widget? trailing;
+  final double? trailingWhitespace;
 
   @override
   Widget build(BuildContext context) {
@@ -466,7 +466,7 @@ class MessageItem extends StatelessWidget {
         isRead: message.flags.contains(MessageFlag.read),
         child: Column(children: [
           MessageWithPossibleSender(item: item),
-          if (trailing != null && item.isLastInBlock) trailing!,
+          if (trailingWhitespace != null && item.isLastInBlock) SizedBox(height: trailingWhitespace!),
         ])));
   }
 }