Menus: Allow empty taxonomy terms to be surfaced when searching for items.

This brings the behaviour inline with that of browsing terms or using the All Items tab, which correctly shows empty terms.

Props birgire, audrasjb

Fixes #45298
Built from https://develop.svn.wordpress.org/trunk@47747


git-svn-id: https://core.svn.wordpress.org/trunk@47523 1a063a9b-81f0-0310-95a4-ce76da25c4cd