Should all singleton types be constructible with T()?

[LilithHafner]

Trivial singleton types declared with

struct T end

already support T(). Nontrivial singleton types such as Tuple{Nothing} and ReverseOrdering{ForwardOrdering} do not. Nevertheless, the only instance of Tuple{Nothing} is (nothing,). Should we return that instance rather than throw on T() for all singleton types T?

julia> Base.issingletontype(Tuple{Nothing})
true

julia> Tuple{Nothing}() # Present
ERROR: MethodError: no method matching Tuple{Nothing}()
[...]

julia> Tuple{Nothing}() # Proposal
(nothing,)

This PR is a special case of this.