There are a possibility that faulty font cache may be stored in the 'kirki_downloaded_font_files' option.

[sebastianthulin]

Issue description:

There are a possibility that faulty font cache may be stored in the 'kirki_downloaded_font_files' option. This option is not checked before store, and will not be updated with correct values later on.

A missing font cache, will result in a GDPR issue since the fonts are linked to google servers.

In my case. "/OK" string has been stored in the option. It might be suitable to do a value check here:
https://github.com/themeum/kirki/blob/master/kirki-packages/module-webfonts/src/Webfonts/Downloader.php#L125

if ( $change && is_array( $stored ) && !empty( $stored ) ) {
update_option( 'kirki_downloaded_font_files', $stored );
}

Possibly, also in the below scope (to be able to mitigate installs that already have stored faulty values).
https://github.com/themeum/kirki/blob/master/kirki-packages/module-webfonts/src/Webfonts/Downloader.php#L68

$stored     = get_option( 'kirki_downloaded_font_files', array() );
if(!is_array($stored)) {
delete_option('kirki_downloaded_font_files'); 
$stored = array();
}

Version used:

5.0.0

Using theme_mods or options?

Not relevant.