FIX: fqdn table placeholder (#299)

frontend/src/components/FQDNTable.vue

 <template>
   <Loading :data="fqdns">
-    <div class="fqdn-table">
+    <div class="fqdn-table" v-if="fqdns">
       <FilterInput v-model="filter"></FilterInput>
       <b-pagination v-if="fqdns.length > per_page && filter === ''" :total-rows="fqdns.length"
                     :per-page="per_page" v-model="current_page" align="center"></b-pagination>
-      <b-table :per-page="per_page" :current-page="current_page" responsive class="shadow" :items="fqdns"
+      <b-table :show-empty="true" empty-text="Keine FQDNs vorhanden." :per-page="per_page" :current-page="current_page" responsive class="shadow" :items="fqdns"
                :fields="fqdn_list_fields" :filter="filter">
         <template v-slot:cell(value)="data">
           <router-link class="nowrap" :to="'/dnsvs/fqdns/'+data.value" v-if="data.item.is_nonterminal">