return empty object if group property is null

Author: abhimediratta

dev/Dev.vue

@@ -4,7 +4,6 @@
       search-input-class="custom-input-class"
       :reduce-display-property="option => option.name"
       :reduce-value-property="option => option.id"
-      :reduceGroupProperty="option => option.group"
       ref="multi"
       highlight-diff
       @diff-changed="handleChange" />

dist/vue-multi-select-listbox.css

@@ -209,7 +209,7 @@ export default {
         return groupedItems;
       }
 
-      return null;
+      return {};
     },
 
     sortedGroupKeys() {