fix(minifyStyles): pass empty arrays for usage

[SethFalco]

CSSO accepts a usage argument, which allows better optimizations since it has the context of the document. This is provided in separate arrays for tags, classes, ids, etc.

Reference: https://github.com/css/csso#usage-data

However, if CSSO has undefined/null for one of these, it interprets that as unknown/no usage data provided, so it's not safe to remove those things.

For example, if CSSO receives undefined for class usage, then it won't remove any classes because it doesn't know whether they're used or not. We have to pass an empty array to tell it no classes have been used in the document.

Reference: https://github.com/css/csso/blob/master/test/usage.js#L81

Also refactors the minifyStyles plugin while I was working here, and removes the style node entirely if it's empty after we're done with it.

Metrics

With only preset-default enabled.

SVG	time	% reduced	final size	borked
Original			2.095 KiB
v3.0.2	22 ms	18.4%	1.709 KiB
main (197be56)	22 ms	18.4%	1.709 KiB
this branch	18 ms	29.2%	1.482 KiB

Related

• Closes Stopped inlining st* classes? #1589