Return `nothing` or the struct in macro struct definitions

[jakobjpeters]

This is just to be a little more friendly in the REPL :)

julia> @kwarg struct A end # expected `A` or `nothing`
kwarg (generic function with 24 methods)

julia> @kwarg struct B # expected `B` or `nothing`
           x = 1
       end
fielddefaults (generic function with 18 methods)

julia> struct C end # returns `nothing`

julia> @kwdef struct D # returns `D`
           x = 2
       end
D