fix: delete_node_by_mem_cube_id

[wustzdy]

Pre-submission checklist | 提交前检查

• I have searched existing issues and this hasn't been mentioned before | 我已搜索现有问题，确认此问题尚未被提及
• I have read the project documentation and confirmed this issue doesn't already exist | 我已阅读项目文档并确认此问题尚未存在
• This issue is specific to MemOS and not a general software issue | 该问题是针对 MemOS 的，而不是一般软件问题

Bug Description | 问题描述

In polardb.py, the soft-delete logic in delete_node_by_mem_cube_id should only proceed if delete_record_id is NULL.

How to Reproduce | 如何重现

the messages are not arranged in an alternating "user-assistant" order.

Environment | 环境信息

Python Version: 3.11
MemOS Version: 2.0.4

Additional Context | 其他信息

No response

Willingness to Implement | 实现意愿

• I'm willing to implement this myself | 我愿意自己解决
• I would like someone else to implement this | 我希望其他人来解决