fix: potential duplicate iteration keys at dropdown (#249)

Author: luisDanielRoviraContreras

lib/default-theme/DropdownLink.vue

@@ -8,8 +8,8 @@
       <ul class="nav-dropdown" v-show="open">
         <li
         class="dropdown-item"
-        v-for="subItem in item.items"
-        :key="subItem.link">
+        v-for="(subItem, index) in item.items"
+        :key="subItem.link || index">
           <h4 v-if="subItem.type === 'links'">{{ subItem.text }}</h4>
           <ul class="dropdown-subitem-wrapper" v-if="subItem.type === 'links'">
             <li