`packageJson` is never `undefined`

The line checks for `packageJson` being falsy:

https://github.com/netlify/netlify-plugin-nextjs/blob/2849dc5f7c57e9fd827a939e067a871e4cb487b1/index.js#L21

However, when a site has no `package.json`, `packageJson` is an empty object. Therefore, `Object.keys(packageJson).length === 0` should be used instead.