Allow .V to be null in emit_unionmove

[Keno]

We generally handle the case where the .V of a split union is as small
as the active element of the union. If the active element happens to be
a ghost is thus seems reasonable for the split union to not have any storage
at all. Such a union is generated e.g. if we widen from an all-ghost split
union to one that includes sized elements, e.g. in the following example:

function foo()
    x = (1, 2)
    if rand() < 0.5
        if rand() < 0.5
            y = nothing
        else
            y = missing
        end
        x = y
    end
    x
end

Fixes #28208