UnzipArrays doesn't handle arrays of Z

I'm in the process of fixing tuple handling (hopefully once and for all), and I tripped across this which seems to be a problem with `UnzipArrays.`

Corresponds to test `p13g`.

What happens is that it drops the array entirely, leaving us with an empty list of program results:

``` Haskell
unzipETups, output was:
================================================================================

Prog {progBinds = [ProgBind tmp_0
(TArray 1 (TTuple []))
(SubBinds {subnames = [], arrsize = Just TrivConst 1},
(Nothing, KnownSize [1]))
(Right Generate (EConst (I 1))
(Lam1 (flatidx0, TInt) (ETuple []))),
ProgBind
(TTuple [])
(SubBinds {subnames = [], arrsize = Nothing},
(Nothing, UnknownSize))
(Left ETuple [])],
progResults = WithShapesUnzipped [(tmp_0, [])],
progType = TArray 1 (TTuple []),
uniqueCounter = 2,
typeEnv = [(tmp_0, TArray 1 (TTuple [])),(tmp_0_shape, TTuple [])]}

unzipArrays, output was:
================================================================================

Prog {progBinds = [ProgBind
(TArray 1 (TTuple []))
(OpInputs [],
(SubBinds {subnames = [], arrsize = Just TrivConst 1},
(Nothing, KnownSize [1])))
(Right Generate (EConst (I 1))
(Lam1 (flatidx0, TInt) (ETuple []))),
ProgBind
(TTuple [])
(OpInputs [],
(SubBinds {subnames = [], arrsize = Nothing},
(Nothing, UnknownSize)))
(Left ETuple [])],
progResults = WithShapesUnzipped [],
progType = TArray 1 (TTuple []),
uniqueCounter = 2,
typeEnv = [(tmp_0, TArray 1 (TTuple [])),(tmp_0_shape, TTuple [])]}
```