Allow `currentIndex` to be null in SidebarItems

[Vorkytaka]

Feature Request:

The SidebarItems widget in the macos_ui package currently requires currentIndex to always be a non-null value between 0 and the total item count, forcing a selection at all times.

Use Case:

When implementing a search function with dynamically updated items, it may be preferable to have no item selected by default (for example, when selected item is out of search). Allowing currentIndex to be null would better reflect an unselected state during such updates.

Proposed Solution:

1. Enable currentIndex to accept null.
2. Adjust widget logic to handle the null state so no item is highlighted.