Multisite module: aclWhereClause allows no groups when site group has no children

If a site group has no children, the call to _multisite_get_all_child_groups( $groupID ) in multisite_civicrm_aclWhereClause returns no childOrgs, so no join/where clause is generated. Consequently a user without view/edit all contacts perm cannot see any contacts.

The attached patch fixes the problem in my case but should be reviewed to make sure it's doing the right thing for other cases.

Dave J