Previously if you constructed an array handle without allocating it, you
would get an error if you tried to use the array as input. This
conflicted with some recent changes to accept empty vectors.

Now when you try to use an unallocated ArrayHandle as input (calling
PrepareForInput or PrepareForInPlace), it internally calls Allocate(0)
(to establish internal state) and sets up a valid execution ArrayPortal
of size 0.