修复下拉列表清除选中抛值问题

src/components/dropdown-list/dropdown-list.vue

@@ -196,7 +196,7 @@ export default class DropDownList extends Vue {
             val = tempVal.length >0?tempVal[0].value:null;
         }
         const type: string = this.$util.typeOf(val);
-        val = Object.is(type, 'null') || Object.is(type, 'undefined') ? undefined : val;
+        val = Object.is(type, 'null') || Object.is(type, 'undefined') ? '' : val;
         this.$emit('change', val);
     }