Functor types aren't written correctly when empty params

[giltho]

Hi !
This is a small thing but I have modules that have side-effect behaviours, and a functor to instantiate them.

module Make () = struct ... end

However, odoc doesn't seem to be handling that well for some reason :

It should be

module BasicIdentifier : functor () -> S_with_stringify with type t = int

Somehow the arrow disappears.
This doesn't happen if I add a dummy parameter

module BasicIdentifier : functor (Empty : sig end) -> S_with_stringify with type t = int

[giltho]

Unfortunately, I don't have time to understand the internals of odoc to fix this, but if someone has time and doesn't know how it works, my first instinct would be to look here where the behaviour of the Unit functor param is different

[jonludlam]

Maybe here: https://github.com/ocaml/odoc/blob/master/src/html/generator.ml#L1671 in contrast to here: https://github.com/ocaml/odoc/blob/master/src/html/generator.ml#L1687 - note the lack of Syntax.Type.arrow