fix(#6080): 头像上传更新后头像不更新，使用 ?? 替代 || 修复空字符串判断

client/src/views/Profile/index.vue

@@ -6,7 +6,7 @@
       <el-col :span="8">
         <div class="page-card profile-card">
           <div class="avatar-section">
-            <el-avatar :size="100" :src="authStore.user?.avatarUrl || undefined">
+            <el-avatar :size="100" :src="authStore.user?.avatarUrl ?? undefined">
               {{ authStore.user?.nickname?.[0] || authStore.user?.username?.[0] }}
             </el-avatar>
             <el-upload