Empty types rhs

[EmileTrotignon]

Base.Nothing.t (https://ocaml.org/p/base/v0.16.3/doc/Base/Nothing/index.html) should be displayed as such :

type Base.Nothing.t = |

it is displayed like that :

type Base.Nothing.t = 

[EmileTrotignon]

This should actually be done in odoc, which is a larger codebase than sherlodoc and might a bit intimidating, however I think in that case it does not require to understand the large odoc codebase, and therefore this is still a good first issue.

The piece of code to fix is here, don't hesitate to ask questions about it.