fix(source-maps): resolve empty sourceRoot sources against the map URL (#8107)

webpack 5 and tsc write "sourceRoot": "" for map-relative sources. Since
5f702dd6c the shared join added "/" for any non-null root, so these
sources resolved from the URL root: rxjs's ../../../src/internal/Observable.ts
under /npm/rxjs@7.8.1/dist/cjs/internal/ became /src/internal/Observable.ts,
and xterm's webpack://xterm/./src/... became https://host/webpack://xterm/....

Treat an empty root like a null root in sourceMapReference, the single join
used by trace-web-source-location, analyze-javascript-application and the
web source-map fetcher. Chrome, Firefox and trace-mapping skip an empty root,
and tc39/ecma426#183 wrote the current steps to match them. Non-empty roots,
including "/" and repeated slashes (#6726), keep their existing behavior.

Fixes #8106

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/domain/javascript/javascriptSourceMapPaths.ts

@@ -57,11 +57,19 @@ export const resolveJavaScriptSourceMapUrl = (
   );
 };
 
+/**
+ * Prefix a source with its declared root before URL resolution. An empty root
+ * adds no prefix: webpack 5 and tsc emit `"sourceRoot": ""` for map-relative
+ * sources. Chrome, Firefox and trace-mapping all skip it, and ECMA-426's
+ * DecodeSourceMapSources steps were written to match them (tc39/ecma426#183).
+ * Only a non-empty root gains a `/` separator, so repeated slashes stay
+ * significant.
+ */
 const sourceMapReference = (
   source: string,
   sourceRoot: string | null | undefined,
 ): string => {
-  if (sourceRoot == null) return source;
+  if (sourceRoot == null || sourceRoot === "") return source;
   return `${sourceRoot}${sourceRoot.endsWith("/") ? "" : "/"}${source}`;
 };