Attribute to indicate result is not null when parameter is true?

[SoftCircuits]

I have something like the following method.

public Node? GetLastNode(bool createNewIfEmpty = false)
{
    // Return last node if any
    if (Nodes.Count > 0)
        return Nodes[Nodes.Count - 1];

    // Return a new appended node, if requested
    if (createNewIfEmpty)
    {
        Nodes.Add(new Node());
        return Nodes[0];
    }

    // Otherwise, return null
    return null;
}

With nullable reference types on, is there any attribute (or other way) to specify that this method never returns null as long as the createNewIfEmpty parameter is true?

If there is no way, should there be?

[RikkiGibson]

I do not think there is a way currently. I think it's necessary to declare a different method which returns not-null.