xarray.DataArray.str.cat() doesn't work on chunked data

What happened?

I was trying to concatenate some DataArrays of strings, and it kept just returning the first DataArray without any changes.

What did you expect to happen?

I was expecting it to just provide the strings, concatenated together with the spearator between them.

Minimal Complete Verifiable Example

da = xr.DataArray(
    np.zeros((2, 2)).astype(str),
    coords={'x':np.arange(2), 'y': np.arange(2)},
    dims=['x', 'y'])

dac = da.chunk()

print((da == dac).values.all())
print((da.str.cat(da, sep='--') == dac.str.cat(dac, sep='--')).values.all())
print((da.str.cat(da, sep='--') == dac.compute().str.cat(dac.compute(), sep='--')).values.all())

>>> True
>>> False
>>> True

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

Start at the xarray.DataArray.str.cat() entry point and reproduce the minimal example with both eager and chunked DataArrays. Trace why the chunked result differs from the computed result, then verify that concatenation with sep='--' agrees for both cases and add or update regression coverage if the relevant test location is found.