How to fix "Your custom PostCSS configuration must export a `plugins` key."

I updated an old Next.js app and when I ran npm run dev I had this error:

error - ./node_modules/next/dist/build/webpack/loaders/css-loader/src/index.js??ruleSet[1].rules[2].oneOf[8].use[1]!./node_modules/next/dist/build/webpack/loaders/postcss-loader/src/index.js??ruleSet[1].rules[2].oneOf[8].use[2]!./styles/globals.css
Error: Your custom PostCSS configuration must export a `plugins` key.

I added a postcss.config.json file with this content:

{
  "plugins": []
}

and the app started working again.