Require a non-0 number of non-empty paths when deserializing routes

Require a non-0 number of non-empty paths when deserializing routes

When we read a `Route` (or a list of `RouteHop`s), we should never
have zero paths or zero `RouteHop`s in a path. As such, its fine to
simply reject these at deserialization-time. Technically this could
lead to something which we can generate not round-trip'ing
serialization, but that seems okay here.