When no group mapping is explicitly provided the XWiki initial groups should be preserved

The initial groups (xwiki.users.initialgroups) are removed immediately by the group synchronisation.

This fix hardcodes the xwiki.users.initialgroups configuration value. A better fix needs this value to be exported from XWiki.class.

Pull request: Initial groups are overwritten by OIDC group synchronisation by ndecker · Pull Request #18 · xwiki-contrib/oidc (github.com)