change empty ProductSpace printing

Author: Jutho

src/spaces/productspace.jl

@@ -68,7 +68,7 @@ dual(P::ProductSpace) = ProductSpace(map(dual, reverse(P.spaces)))
 function Base.show(io::IO, P::ProductSpace{S}) where {S <: ElementarySpace}
     spaces = P.spaces
     if length(spaces) == 0
-        print(io, "ProductSpace{", S, ", 0}")
+        return print(io, "one(", type_repr(S), ")")
     end
     if length(spaces) == 1
         print(io, "ProductSpace")