Do not display search box when there's no packages yet (#28146) (#28159)
Backport #28146 by @yp05327
Before:
![image](https://github.com/go-gitea/gitea/assets/
18380374/
3012f544-7ff5-4ccb-ac80-
ce24d50abe97)
After:
![image](https://github.com/go-gitea/gitea/assets/
18380374/
4084312a-9ac0-4103-8c93-
ea178ae24493)
![image](https://github.com/go-gitea/gitea/assets/
18380374/
3c47d175-0735-476d-8979-
da2bc0a4fc95)
![image](https://github.com/go-gitea/gitea/assets/
18380374/
033c6a81-d1f7-4426-8063-
5793d0b47462)
Co-authored-by: yp05327 <576951401@qq.com>