Customize: Prevent nav menu location assignment checkbox change from increasing height by 1px.

Props obenland.
Amends [41817].
See #42157.
Fixes #42492.

Built from https://develop.svn.wordpress.org/trunk@42152


git-svn-id: https://core.svn.wordpress.org/trunk@41983 1a063a9b-81f0-0310-95a4-ce76da25c4cd