Boost: Relative URLs break critical css generation

[dilirity]

p1726628647463949-slack-C016BBAFHHS

If a source provider has a relative URL, the URL constructor throws an error because it's an invalid URL and breaks the whole generation process. It shouldn't - it should probably show the error under advanced recommendation but it should not break the whole process.

The PHP side that makes sure a relative URL doesn't end up in the source providers will be handled in #39456.

We still need to update the UI to not show a fatal error if a malformed URL reaches critical css. In other words, handle exceptions when the URL is attempted to be created before passing it to the generator (or is it the css generator that throws the error?).