At runtime, some code pass a `null` Cell parameter, for example in `Graph.getEdges`. Previously, the implementation
thrown an error if Cell was null.

Also improve the signature of Graph.isValidAncestor
  - the `cell` parameter accepts `null` values
  - the `recurse` parameter is marked as optional

tbouffard marked this pull request as ready for review

tbouffard deleted the fix/236_isValidAncestor_accept_null_Cell branch