fix(message): 修复对话列表头像和昵称显示逻辑

- 移除头像图片的本地资源兜底逻辑，统一使用网络图片
- 简化昵称显示逻辑，优先显示昵称，否则显示空字符串
- 移除冗余的空值检查和类型转换代码

lib/pages/message/conversation_tab.dart

@@ -109,12 +109,7 @@ class _ConversationTabState extends State<ConversationTab>
                             decoration: BoxDecoration(
                               borderRadius: BorderRadius.circular(28),
                               image: DecorationImage(
-                                image: (userInfo?.avatarUrl?.isNotEmpty ?? false)
-                                    ? NetworkImage(userInfo!.avatarUrl!)
-                                    : const AssetImage(
-                                            Assets.imagesAvatarsExample,
-                                          )
-                                          as ImageProvider,
+                                image: NetworkImage(userInfo!.avatarUrl!),
                                 fit: BoxFit.cover,
                               ),
                             ),
@@ -131,9 +126,7 @@ class _ConversationTabState extends State<ConversationTab>
                                   children: [
                                     Expanded(
                                       child: Text(
-                                        (userInfo?.nickName?.isNotEmpty ?? false)
-                                            ? userInfo!.nickName!
-                                            : conversation.id, // 如果没有昵称，显示用户ID
+                                        userInfo.nickName ?? '', // 如果没有昵称，显示用户ID
                                         style: const TextStyle(
                                           fontSize: 16,
                                           fontWeight: FontWeight.w500,