Customizer: Don't override `Section.isContextuallyActive()` in `SidebarSection`.

This fixes a bug where empty widget areas get deactivated in the Customizer.

fixes #30378.
see #30235.
props westonruter.

git-svn-id: https://develop.svn.wordpress.org/trunk@30552 602fd350-edb4-49c9-b593-d223f7449a82