max() over empty list in Gingleator

[AustinLBuchanan]

In Gingleator, if all districts become majority-minority, then an error occurs on line 144 of reward_partial_dist(). In this case, it tries to take the max( ) over an empty list, and the code breaks. The same issue could arise on line 171 of reward_next_highest_close().

GerryChain/gerrychain/optimization/gingleator.py

Line 144 in a83acf5

next_dist = max(i for i in dist_percs if i < threshold)

GerryChain/gerrychain/optimization/gingleator.py

Line 171 in a83acf5

next_dist = max(i for i in dist_percs if i < threshold)

It would be great if GerryChain could terminate gracefully in such circumstances, either terminating the chain early because a global max has been found, or finishing the chain as intended but without returning an exception. Either seems reasonable to me.