Failures when trying to access /wikis/query endpoint

The endpoint /wikis/query always conflicted with endpoint /wikis/{wikiName} (created XWIKI-24146 for that), but XWIKI-17936 made it worst by setting the wiki to (the often not existing) wiki "query" when calling it.

The path /wikis/query was a very bad idea (should have been /query probably, since it's not returning wiki descriptors) but it's hard to change now.