refactor: 将默认值“空数据”替换为“N/A”

统一将默认值“空数据”替换为“N/A”，提高代码一致性和可读性

@@ -17,10 +17,10 @@
     <div class="flex-1 p-3 flex flex-col justify-between">
       <div>
         <h3 class="font-medium text-sm mb-1 line-clamp-2">{{ course.title }}</h3>
-        <div class="text-gray-500 text-xs">{{ course.subtitle || '空数据' }}</div>
+        <div class="text-gray-500 text-xs">{{ course.subtitle || 'N/A' }}</div>
       </div>
       <div class="flex justify-between items-end mt-1">
-        <div class="text-orange-500 font-semibold">¥{{ course.price || '空数据' }}</div>
+        <div class="text-orange-500 font-semibold">¥{{ course.price || 'N/A' }}</div>
         <div class="text-gray-400 text-xs">
           {{ course.subscribers || '没字段' }}人订阅
         </div>

@@ -10,13 +10,13 @@
           <h1 class="text-2xl text-white font-bold mb-1">{{ course?.title }}</h1>
           <h2 class="text-lg text-white/90">{{ course?.subtitle  || '没有字段' }}</h2>
           <div class="mt-4 flex justify-between items-center">
-            <div class="text-orange-300 font-bold text-2xl">¥{{ course?.price || '空数据' }}</div>
+            <div class="text-orange-300 font-bold text-2xl">¥{{ course?.price || 'N/A' }}</div>
             <div class="bg-orange-500/30 text-orange-100 text-xs px-3 py-1 rounded-full">
               限时优惠
             </div>
           </div>
           <div class="flex justify-between text-xs text-white/80 mt-3">
-            <div>已更新{{ course?.count || '空数据' }}期</div>
+            <div>已更新{{ course?.count || 'N/A' }}期</div>
             <div>{{ course?.subscribers || '没有字段' }}人订阅</div>
           </div>
           <div v-if="course?.expireDate" class="text-xs text-white/80 mt-1">
@@ -73,14 +73,14 @@
                 <li>随堂练习，巩固知识</li>
                 <li>及时反馈，调整教学</li>
               </ul> -->
-              <div v-html="course?.feature || '空数据'"></div>
+              <div v-html="course?.feature || 'N/A'"></div>
             </div>
 
             <div v-if="activeTab === '课程大纲'">
               <div class="space-y-4">
                 <div v-for="(item, index) in displayedSchedule" :key="index" class="border-l-2 border-green-500 pl-3">
                   <h4 class="font-medium text-gray-800">{{ item.title }}</h4>
-                  <p class="text-sm text-gray-600 mt-1">{{ item.duration }}分钟 · {{ item.schedule_time || '空数据' }}个小节</p>
+                  <p class="text-sm text-gray-600 mt-1">{{ item.duration }}分钟 · {{ item.schedule_time || 'N/A' }}个小节</p>
                 </div>
                 <div v-if="course?.schedule?.length > 4" class="flex justify-center mt-4">
                   <button @click="toggleSchedule" class="p-2 rounded-full hover:bg-green-50 text-green-600 hover:text-green-700 transition-all duration-300">
@@ -92,13 +92,13 @@
 
             <div v-if="activeTab === '课程亮点'">
               <div class="space-y-3 text-gray-700">
-                <div v-html="course?.highlights || '空数据'"></div>
+                <div v-html="course?.highlights || 'N/A'"></div>
               </div>
             </div>
 
             <div v-if="activeTab === '学习目标'">
               <div class="space-y-3 text-gray-700">
-                <div v-html="course?.learning_goal || '空数据'"></div>
+                <div v-html="course?.learning_goal || 'N/A'"></div>
               </div>
             </div>
           </div>