`packageJson` is never `undefined`

[ehmicky]

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.

[ehmicky]

Fixed by #29.