ADD: placehlder text for missing sub ous

frontend/src/views/org/OU.vue

@@ -245,6 +245,10 @@
           <div v-for="top_level_ou in ous_by_parent[ou.short_name]" :key="top_level_ou.short_name">
             <OUTreeEntry :ou="top_level_ou" :ous_by_parent="ous_by_parent"/>
           </div>
+          <div v-if="!ous_by_parent[ou.short_name] || ous_by_parent[ou.short_name].length === 0"
+               class="font-italic text-center my-3">
+            <span class="font-italic">Keine untergeordneten Organisationseinheiten vorhanden</span><br/>
+          </div>
         </template>
         <template v-else>
           <div v-for="top_level_ou in root_ous" :key="top_level_ou.short_name">