Namely:

- return `Nothing` instead of failing with the dreaded `*** Exception:
  Prelude.head: empty list` (for `spLength`) or returning
  non-sense (for `sp`) when the source is not connected to the
  destination.

- mention in the docs that negative edge weights will not produce the
  right answer.

- add tests that check that `Nothing` and `Just` are returned when
  they should be for shortest-path operations.