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.
Built from https://develop.svn.wordpress.org/trunk@30552


git-svn-id: http://core.svn.wordpress.org/trunk@30541 1a063a9b-81f0-0310-95a4-ce76da25c4cd