fix: 学员手册没有的时候,隐藏查看学员手册

src/components/learnSysLayout/navigation.vue

@@ -6,7 +6,7 @@
     </div>
     <div class="nav-right">
       <div class="nav-a">
-        <a :href="documentUrl" target="_blank">{{ $t('components.learnSysLayout.navigation.handlbook') }}</a>
+        <a :href="documentUrl" target="_blank" v-if="documentUrl">{{ $t('components.learnSysLayout.navigation.handlbook') }}</a>
         <!-- <a href="https://library.ciis.edu" target="_blank">{{ $t('components.learnSysLayout.navigation.library') }}</a> -->
         <router-link to="/app/other/qa">常见问题与解答</router-link>
       </div>