Fix generation of div:not(.foo) if .foo is never defined

[RobinMalfait]

This PR will fix an issue where some code never got generated.

E.g.:

@layer base {
  div:not(.foo) {
    background-color: pink;
  }
}

<div>I am a div</div>

You would expect that this div has a background color of pink, but that's not the case because we never generated the above css. This is because it contains a class that doesn't exist and therefore we never generated it.
It could happen that you forget about this code, and all of a sudden you use .foo in an unrelated spot. This means that all of a sudden this code doess get generated and now your div is pink.

This PR will fix that by making sure that this gets generated. We already generate non "on-demandable" code (always). For example div {} will always be generated.
To fix this, we ignore everything inside the :not when extracting classes, this results in div {} and doesn't contain any classes thus we generate it. Note, removal of :not() only happens when extract classes, the final result still contains the :not.

Fixes: #7750