Ensure full node stays, if only a partial is used

[RobinMalfait]

Imagine you have this css:

:root, .a {
  color: red;
}

If you never use the a class, then this would never be generated. However, if you have this:

:root { color: red }
.a { color: red }

Then the final result would look like this:

:root { color: red; }

Essentially what this PR does is make sure that we keep the full node if one of the selectors contains non "on-demandable" parts. E.g.: :root.

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Fixes: #5085