fix bug: default-sort-prop is not defined but trigger sort-change event

packages/table/src/table-header.js

@@ -190,6 +190,7 @@ export default {
   },
 
   mounted() {
+    if (this.defaultSortProp) {
       const states = this.store.states;
       states.sortProp = this.defaultSortProp;
       states.sortOrder = this.defaultSortOrder;
@@ -204,6 +205,7 @@ export default {
 
         this.store.commit('changeSortCondition');
       });
+    }
   },
 
   beforeDestroy() {