Incorrect rechunking when input array only has 1 chunk in rechunk_for_blockwise

Example:

```
import dask.array as da
import flox
import numpy as np

# works
assert ((3, 2),) == flox.rechunk_for_blockwise(da.zeros(5, chunks=(1,)), 0, np.array([1, 1, 1, 2, 2]))[1].chunks

# fails
assert ((3, 2),) == flox.rechunk_for_blockwise(da.zeros(5), 0, np.array([1, 1, 1, 2, 2]))[1].chunks
```