In `_wp_json_convert_string()`, when `$use_mb` is `false`, perhaps pass a variable that actually exists to `wp_check_invalid_utf8()`.

Introduced in [30055].
See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30162 602fd350-edb4-49c9-b593-d223f7449a82