Bundled Themes: Twenty Twenty customizer retina_logo should be hidden when no logo is set.

Modifies the Customize setting `Site Identity > Retina logo` so that the option is hidden if there is no logo is set in `Site Identity > Logo`.

Props truongwp.
Fixes #50109.
Built from https://develop.svn.wordpress.org/trunk@47776


git-svn-id: https://core.svn.wordpress.org/trunk@47552 1a063a9b-81f0-0310-95a4-ce76da25c4cd