extract_random_route sometimes extracts zero-length routes!

[Nikronic]

The possibility is very low, because the capacity of the each depot is so big so at least we have 2 customer in each route of depot. But still there is no reason to extract an empty route when there is no empty route in the depot itself.

May be initial_routing is generating bad routes sometimes??? When the capacity was between [0, 100], there were a lot of single customer routes, so maybe caused empty route too. I have changed the random to [100, 200]so now each route approximately has at least twocustomers` so we won't be able to see an empy route again maybe.

I have not added tests for empty route in initial_routing but I have added some tests that checks there should not be any sequential customers with ID=999 which represents null customers. It means if there is an empty route, only a null customer will be added so we it cannot pass this test.