shortcircuit is_node_convex when there is exactly one node

[doug-q]

in is_node_convex we currently have

        if nodes.is_empty() {
            return true;
        }

A graph consisting of exactly one node is always convex so we should instead if nodes.len() <= 1. Convexity checking is expensive, so this optimisation does seem worthwhile. Ref CQCL/hugr#1652