`Dataset.to_array()` throws `IndexError` for empty datasets

What happened?

>>> xr.__version__
'2023.4.2'
>>> xr.Dataset().to_array()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/mnt/qb/work2/goswami0/gkd021/conda/envs/wb/lib/python3.10/site-packages/xarray/core/dataset.py", line 6114, in to_array
    data = duck_array_ops.stack([b.data for b in broadcast_vars], axis=0)
  File "/mnt/qb/work2/goswami0/gkd021/conda/envs/wb/lib/python3.10/site-packages/xarray/core/duck_array_ops.py", line 326, in stack
    xp = get_array_namespace(arrays[0])
IndexError: list index out of range

What did you expect to happen?

The most reasonable way to handle this in my opinion would be to return an empty, i.e. default constructed, xr.DataArray:

>>> xr.DataArray()
<xarray.DataArray ()>
array(nan)

Minimal Complete Verifiable Example

No response

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 in xarray/core/dataset.py at Dataset.to_array, then reproduce the issue with xr.Dataset().to_array() using the example in the report. The change is complete when an empty Dataset returns an empty, default-constructed DataArray instead of raising IndexError, with regression coverage for that case.