XWIKI-9501: Two Admin users are added when creating a new Workspace on a migrated main wiki.

* Fixed. Has been introduced by XWIKI-6275. The group object that contains "XWiki.Admin" had a wrong number. Then EM was not able to merge groups the way we wanted.
Merge pull request #182 from gdelhumeau/XWIKI-9501-2

XWIKI-9501: Two Admin users are added when creating a new Workspace on a migrated main wiki