Ring handler created by ring-serve should never return nil

[jeluard]

Some ring implementation (e.g. aleph) do not interpret a nil returned by a na route match as a 404.

This change allows a no route match to call get-handler with nil as route. Implementer is now free to return a custom response.

[jeluard]

CircleCI tests failed but it doesn't appear to be related with this change?

[domkm]

Thanks!