Bug: 按时间group, 分组名显示 empty

<img width="647" height="258" alt="Image" src="https://github.com/user-attachments/assets/0767ce94-64a2-40d5-835c-186ac2ad1902" />


对比nocodb的表现

<img width="952" height="196" alt="Image" src="https://github.com/user-attachments/assets/438e3601-af36-421c-972e-e470722f0b61" />