Customize: Fix logic for previewing the URL for `nav_menu_item` settings for...

Customize: Fix logic for previewing the URL for `nav_menu_item` settings for terms and post type archives.

Fixes typo in args passed to `get_term_link()` which caused a fatal error due to this call returning a `WP_Error` which was set to `url`. Also fixes never-satisfiable condition for obtaining post type archive URL. Also ensures that `WP_Error` never leaks through as `url` by setting it to an empty string. Adds missing unit tests.

Amends [38991].
See #38114.
Fixes #38945.

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git-svn-id: https://core.svn.wordpress.org/trunk@39305 1a063a9b-81f0-0310-95a4-ce76da25c4cd