fix bug in purgecss extractor
fix bug in purgecss extractor

`content.match` will return `null` on files without any  matches (e.g. empty files). This causes purgecss to fail. Returning an empty array on falsey values fixes this
tailwindlabs/tailwindcss,tailwindcss/tailwindcss,tailwindlabs/tailwindcss,tailwindlabs/tailwindcss