Don't set MemberID on Order record if Member not logged in

In particular, if you overload MembershipCheckoutComponent::setData()
and decide not to call logIn() on a newly created Member and set the
MemberID on the Order record at that point, calls to set MemberID on
the Order in Checkout will override that back to 0, as
Member::currentUserID() would return 0.

To fix this, simply check for Member::currentUser()
before attempting to set the Order->MemberID relationship.