fix(issue events): assignees may be undefined

在 gitlab-13.5.3-ee 中测试发现 issue 类型事件中，assignees 不指定的情况下为 undefined

Author: hanyuzhou2006

app/service/webhook.js

@@ -220,7 +220,7 @@ class WebhookService extends Service {
  return content
  }
 
- async assembleIssueMsq(content, { user, project, repository, object_attributes, assignees, assignee, labels }) {
+ async assembleIssueMsq(content, { user, project, repository, object_attributes, assignees = [], assignee, labels }) {
  const { id: issueId, title, state, action, description, url: issueUrl } = object_attributes || {};
  const { name: projName, web_url, path_with_namespace } = project || {};
  const { name, username } = user || {};
@@ -455,4 +455,4 @@ class WebhookService extends Service {
 
 }
 
-module.exports = WebhookService;
+module.exports = WebhookService;