fix(common) 修复管理员未设置Email主题前缀导致发送邮件失败的问题

apps/common/tasks.py

@@ -24,7 +24,7 @@ def send_mail_async(*args, **kwargs):
     """
     if len(args) == 3:
         args = list(args)
-        args[0] = settings.EMAIL_SUBJECT_PREFIX + args[0]
+        args[0] = (settings.EMAIL_SUBJECT_PREFIX or '') + args[0]
         email_from = settings.EMAIL_FROM or settings.EMAIL_HOST_USER
         args.insert(2, email_from)
         args = tuple(args)