Customize: Fix inability to delete nav menus by preventing `preview` filters...

Customize: Fix inability to delete nav menus by preventing `preview` filters from being added during `customize_save` admin ajax request.

Also prevent setting `nav_menu_locations[...]` values to `NaN` which gets sent as `null`.

Amends [38810].
See #30937.
Fixes #39103.

Built from https://develop.svn.wordpress.org/trunk@39558


git-svn-id: https://core.svn.wordpress.org/trunk@39498 1a063a9b-81f0-0310-95a4-ce76da25c4cd