Non-existing links should have rel=nofollow attribute, since search engines crawlers should not follow those links until target pages are created.
Web crawlers are currently blocked by a robots.txt fallback rule. Adding nofollow attribute for non-existing links may improve/impact visibility (from SEO perspectives), so I'm tagging Product-Analytics as well.
User-agent: * Disallow: /w/
I found a long forgotten bug report on the issue, T21065, marked "Resolved", but failed to find further information about it.