Fix webpack asset patch with '/' as relative root

What does this MR do?

Fix the handling of relative_url_root in lib/utils/webpack.js (see relevant issue).

Why was this needed?

Recent versions of gdk default relative_url_root to /, which causes issues when this is simply concatenated to paths.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides

Links

• Related issue https://gitlab.com/gitlab-org/gitlab-ce/issues/60932.