Empty let moves comments

[brandonchinn178]

Describe the bug
let is allowed to have no bindings, but this causes any comments later in the file to be moved into the let

To Reproduce

foo =
  let
   in 10

-- a comment
bar = 20

gets reformatted to

foo =
  let -- a comment

   in 10

bar = 20

Expected behavior
Should not change

Environment

• OS name + version:
• Version of the code: ormolu-live.tweag.io

Additional context
Add any other context about the problem here.