Fix an empty tuple problem ahead of Julia 1.10

[KevinDCarlson]

This code in pack_limit can lead to bad behavior if enough stuff is empty, as in the test on line 352 of CSets. This breaks in Julia 1.10 because Tuple{Union{}} is going to become illegal. Pretty sure the map just needs to be replaced with an explicit comprehension into VarFunctions, just not going to get to it tonight.

alim = NamedTuple(Dict(map(attrtypes(S)) do at 
      T = attrtype_type(Y, at)
      apx = VarSet{T}(nparts(Y, at))
      at => Multispan(apx, map(enumerate(cone_objects(diagram))) do (i, X) 
        v = map(parts(Y,at)) do p 
          f, c, j = var_reference(Y, at, p)
          X[legs(limits[c])[i](j), f]
        end
        VarFunction{T}(v, FinSet(nparts(X, at)))
      end)
    end))