fixed bug in interaction between node sizes and unused_color=None

[boothby]

While bughunting on Friday, I was attempting to use overlapped embeddings to highlight qubits involved in a bad embedding (without showing the unused qubits). It didn't work because of how the node_size parameter is being handled, so I tracked it down and fixed it. This PR is missing unit tests, and I suspect that other parameters might be getting mangled.