Fix handling of unit values -- repackAcc broken for Array of Z

In particular for types like `Acc (Array (Z:.Int) Z)`.

It gets this:

```
run test 20/67 p13g2:: [Failed]
ERROR: concatAccArrays: Cannot zip an empty list of AccArrays (don't know dimension)
```

I added a unit test to `backend-kit/tests/UnitTests.hs` that reproduces this.