fix(utils): Add null check to parseCrossOriginStylesheet, closes #5074

[dcbroad3]

If a null href is passed to parseCrossOriginStylesheet, the function will currently convert that to the string "null" and attempt to load that href. This PR adds a null check so that the function instead returns early if a nullish href is passed, similar to how parseSameOriginStylesheet returns early if a sheet includes no rules.

Closes:
#5074

[Copilot]

Pull request overview

This PR addresses axe-core issue #5074 by preventing parseCrossOriginStylesheet from attempting to fetch a stylesheet when the provided href/URL is nullish, avoiding erroneous network requests like "null".

Changes:

• Add a nullish check in parseCrossOriginStylesheet to return early when url == null.
• Add a unit test intended to cover the nullish-URL case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
lib/core/utils/parse-crossorigin-stylesheet.js	Adds early return when the cross-origin stylesheet URL is nullish.
test/core/utils/parse-crossorigin-stylesheet.js	Adds coverage for the nullish-URL behavior.

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[dcbroad3]

I am following the pattern of the early return in parseSameOriginStylesheet if rules is falsy, but let me know if a different result should be returned (or if the function should do an early reject instead?)