fix: improve formatting and add JSON validation in McpServersDialog

--bug=1060753 --user=刘瑞斌 【应用】AI对话节点的MCP使用自定义方式，配置格式错误依然可以保存成功 https://www.tapd.cn/62980211/s/1760128

ui/src/views/application/component/McpServersDialog.vue

@@ -154,7 +154,7 @@ const submit = () => {
   paramFormRef.value.validate((valid: any) => {
     if (valid) {
       try {
-        JSON.parse(form.value.mcp_servers)
+        JSON.parse(form.value.mcp_servers || '{}')
       } catch (e) {
         MsgError(t('views.applicationWorkflow.nodes.mcpNode.mcpServerTip'))
         return