Implement _show_leaflike(::Tuple{}) to handle parameters consisting of an empty Tuple

[JoshuaBillson]

Given a layer with a parameter of type Tuple{}, the following error is thrown when attempting to show it:

ERROR: MethodError: _show_leaflike(::Tuple{}) is ambiguous.

Candidates:
  _show_leaflike(::Tuple{Vararg{AbstractArray}})
    @ Flux ~/.julia/packages/Flux/3711C/src/layers/show.jl:73
  _show_leaflike(::Tuple{Vararg{Number}})
    @ Flux ~/.julia/packages/Flux/3711C/src/layers/show.jl:72

Possible fix, define
  _show_leaflike(::Tuple{})

I've added the method _show_leaflike(::Tuple{}) = true, which appears to resolve the issue.

I don't know if it's appropriate to add new documentation and an entry in NEWS.md for this, but I can do so if requested.

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable