Group admin: "No group found with this ID" block cannot be reached

[boonebgorges]

The logic looks like this:

$group = groups_get_group( 'group_id=' . (int) $_GET['gid'] );
if ( ! empty( $group ) ) {

} else {

}

But groups_get_group() never returns empty. It always returns a group object.

[boonebgorges]

In 10672:

Group admin: Improve logic when deciding whether to show "No group found" message.

See #6911.

Fixes #6975.