bug: fails to find `bestMatch` under some situations

`bestMatch` is not always located before [this branch](https://github.com/faceyspacey/extract-css-chunks-webpack-plugin/blob/b9cfc9cef4841802a3f2e61f6493ad745cfefb98/src/index.js#L505) is triggered.

As a result, there's a `Cannot read property 'pop' of undefined` [error](https://github.com/zeit/next.js/issues/10161) fired.

Here's a really small project that reproduces the issue:
https://github.com/Timer/extract-css-chunks-webpack-plugin-bug

This same exact input works with [`mini-css-extract-plugin`](https://github.com/webpack-contrib/mini-css-extract-plugin), so it might be worth finding where logic diverges!

Node: v12.14.1
`extract-css-chunks-webpack-plugin`: 4.7.4