[kunal649]

Why:
fetch('') is instrumented, but xhr.open('GET', '') was not, and once an empty URL request is instrumented its breadcrumb shows url: "". This PR fixes both

• xhr.ts bailed on !url, which treats an empty string like an unparseable argument, so empty-URL XHRs got no breadcrumb and no span. It now only bails on url === undefined, so XHR matches fetch.
• The fetch and XHR breadcrumb handlers now show the page URL for an empty URL instead of "". Query and fragment are stripped with stripUrlQueryAndFragment, so params like ?token= from the page URL don't end up in breadcrumbs. Since an empty URL resolves to the page URL itself, the helper uses location.href directly instead of calling new URL() (bcoz its throws for hrefs like about:srcdoc). without a location object (e.g. workers), nothing changes.

Only empty URLs are rewritten, absolute and relative URLs keep their current breadcrumb behaviour.

How to review:
Start with:

• packages/browser-utils/src/instrumentation/xhr.ts
• packages/browser/src/integrations/breadcrumbs.ts

and the regression coverage in:

• packages/browser-utils/test/instrumentation/xhr.test.ts
• packages/browser/test/integrations/breadcrumbs.test.ts

The key things to check are that only empty URLs are rewritten, and that the query and fragment are stripped from the page URL in all three breadcrumb paths (fetch, failed fetch, XHR).

Unsure about:
Whether relative URLs (like /api/foo?x=1) should also be resolved against location.href and stripped in breadcrumbs. I kept this limited to empty URLs so the change stays scoped, but yeah I'm happy to widen it.

Ref #13168