fix(mp): App instance maybe no triggerEvent method

packages/vue-cli-plugin-uni/packages/mp-vue/dist/mp.runtime.esm.js

@@ -5826,10 +5826,13 @@ function internalMixin(Vue) {
 
   Vue.prototype.$emit = function(event) {
     if (this.$scope && event) {
-      (this.$scope['_triggerEvent'] || this.$scope['triggerEvent']).call(this.$scope, event, {
+      var triggerEvent = this.$scope['_triggerEvent'] || this.$scope['triggerEvent'];
+      if (triggerEvent) {
+        triggerEvent.call(this.$scope, event, {
           __args__: toArray(arguments, 1)
         });
       }
+    }
     return oldEmit.apply(this, arguments)
   };