REQ-3345: 如果聊天消息只有一个人的情况下，同步发送

im-center-server/src/main/java/cn/axzo/im/controller/MessageController.java

@@ -264,6 +264,7 @@ public class MessageController implements MessageApi {
             }
         }
         int receiverSize = request.receivePersonsOrEmpty().size() + request.imReceiveAccountsOrEmpty().size();
+        BizAssertions.assertTrue(receiverSize >= 1, "接收人不能为空");
         boolean syncSend = request.isTrySyncSend() && receiverSize == 1;
         MessageTask.BizData bizData = MessageTask.BizData.builder()
                 .messageBody(JSON.toJSONString(request.getMessageBody()))