Degree assortativity functions don't handle empty hypergraphs

[nwlandry]

Returns IndexError.

[leotrs]

Now that I think of it, probably nothing handles empty hypergraphs...

[maximelucas]

By empty do you mean (1) no nodes and no edges, or (2) just no edges? Do we want to return None for (1) but 0 for (2)?

[leotrs]

Agreed. No edges should have a numerical answer, perhaps even 0, inf, or NaN.

No nodes at all should return None, or raise an error.