open_zarr: concat_characters has no effect when dtype=U1

What happened:

It appears that either to_zarr or from_zarr is incorrectly concatenating the trailing dimension of single byte/character arrays and dropping the last dimension:

import xarray as xr
import numpy as np
xr.set_options(display_style='text')

chrs = np.array([
    ['A', 'B'],
    ['C', 'D'],
    ['E', 'F'],
], dtype='S1')
ds = xr.Dataset(dict(x=(('dim0', 'dim1'), chrs)))
ds.x
<xarray.DataArray 'x' (dim0: 3, dim1: 2)>
array([[b'A', b'B'],
       [b'C', b'D'],
       [b'E', b'F']], dtype='|S1')
Dimensions without coordinates: dim0, dim1

ds.to_zarr('/tmp/test.zarr', mode='w')
xr.open_zarr('/tmp/test.zarr').x.compute()
# The second dimension is lost and the values end up being concatenated
<xarray.DataArray 'x' (dim0: 3)>
array([b'AB', b'CD', b'EF'], dtype='|S2')
Dimensions without coordinates: dim0

For N columns in a 2D array, you end up with an "|SN" 1D array. When using say "S2" or any fixed-length greater than 1, it doesn't happen.

Interestingly though, it only affects the trailing dimension. I.e. if you use 3 dimensions, you get a 2D result with the 3rd dimension dropped:

chrs = np.array([[
    ['A', 'B'],
    ['C', 'D'],
    ['E', 'F'],
]], dtype='S1')
ds = xr.Dataset(dict(x=(('dim0', 'dim1', 'dim2'), chrs)))
ds
<xarray.Dataset>
Dimensions:  (dim0: 1, dim1: 3, dim2: 2)
Dimensions without coordinates: dim0, dim1, dim2
Data variables:
    x        (dim0, dim1, dim2) |S1 b'A' b'B' b'C' b'D' b'E' b'F'

ds.to_zarr('/tmp/test.zarr', mode='w')
xr.open_zarr('/tmp/test.zarr').x.compute()
# `dim2` is gone and the data concatenated to `dim1`
<xarray.DataArray 'x' (dim0: 1, dim1: 3)>
array([[b'AB', b'CD', b'EF']], dtype='|S2')
Dimensions without coordinates: dim0, dim1

In short, this only affects the "S1" data type. "U1" is fine as is "SN" where N > 1.

Environment:

Reproduce the issue with the provided to_zarr and open_zarr examples, comparing S1, SN, and U1 arrays. Trace the serialization and deserialization path involved in those entry points; done means the trailing dimension and character values are preserved for S1 arrays.