fix: 修复嵌入应用的指定回复节点回复None的缺陷

--bug=1049643 --user=王孝刚 【应用】简单应用输出节点连接多个执行回复节点，回复节点引用简单应用的输出结果变量，执行后回复结果为None https://www.tapd.cn/57709429/s/1621456

apps/application/flow/step_node/application_node/impl/base_application_node.py

@@ -18,6 +18,7 @@ def _write_context(node_variable: Dict, workflow_variable: Dict, node: INode, wo
     node.context['message_tokens'] = result.get('usage', {}).get('prompt_tokens', 0)
     node.context['answer_tokens'] = result.get('usage', {}).get('completion_tokens', 0)
     node.context['answer'] = answer
+    node.context['result'] = answer
     node.context['question'] = node_variable['question']
     node.context['run_time'] = time.time() - node.context['start_time']
     if workflow.is_result(node, NodeResult(node_variable, workflow_variable)):

ui/src/workflow/icons/application-node-icon.vue

@@ -1,12 +1,12 @@
 <template>
   <AppAvatar
-    v-if="isAppIcon(item.icon)"
+    v-if="isAppIcon(item?.icon)"
     shape="square"
     :size="32"
     style="background: none"
     class="mr-8"
   >
-    <img :src="item.icon" alt="" />
+    <img :src="item?.icon" alt="" />
   </AppAvatar>
   <AppAvatar
     v-else-if="item?.name"