fix(website): empty css cache response generates invalid font-face

[ayuhito]

When converting an undefined variable to string later in the @font-face generate, it actually converts it to "undefined" as a string which mangles the CSS into something like undefined@font-face {...}. This broke loading fonts in some areas of the website.

Also a tiny styling fix to the variable filters checkbox that broke the flex gap between the checkbox and text.

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Latest commit: 1c9a30c

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[github-actions]

Deployed to staging: https://staging.fontsource.org/