Fix assortativity_coeff in graphs.utilities

[CharJon]

Using this graph in example/graph_properties.py I got:
/home/jochar/miniconda3/envs/geco/lib/python3.8/site-packages/networkx/algorithms/assortativity/correlation.py:287: RuntimeWarning: invalid value encountered in double_scalars return (xy * (M - ab)).sum() / numpy.sqrt(vara * varb)

[mmghannam]

I could not reproduce the error, I loaded the graph and called nx.degree_assortativity_coefficient() on it and -0.020197899023141477 was the result. I looked up the error and it seems like it's a division by zero error because of numerical instability (numbers are too small so they are approximated to zero), I found the info here.

[CharJon]

I could not reproduce the error, I loaded the graph and called nx.degree_assortativity_coefficient() on it and -0.020197899023141477 was the result. I looked up the error and it seems like it's a division by zero error because of numerical instability (numbers are too small so they are approximated to zero), I found the info here.

As discussed: We should try to catch this error and set the value to None. And possibly print a warning.