Auto merge of #17537 - jyc:default-namespace-serialization, r=emilio

Some fixes to selector serialization re: namespaces and universal selector

- Fix eliding default namespace when serializing
- Fix shortest serialization property when namespace prefix is `*|` and there is no default namespace
- Omit universal selector when serializing to match `cssom/serialize-namespaced-type-selectors` (again so we get the shortest serialization)

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- [X] These changes is part of a series to fix #17182

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I'd like to land #17501 first, because it allows some tests for this to work.

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