Change behavior of keep_attrs in xr.where when x is a scalar

[jsignell]

• Closes Inconsistent behavior of xarray.where for coordinate attributes #10982
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

Based on how keep_attrs=False behaves in other methods, I think it should drop all attrs.

Demo

import xarray as xr
import numpy as np

rng = np.random.default_rng(1)
arr = xr.DataArray(
    rng.integers(0, 3, (3, 3)), coords={"a": np.arange(3), "b": np.arange(3)}
)
arr["a"].attrs = {"foo": "bar", "baz": 1}
ds = xr.Dataset({"arr": arr})

print(xr.where(ds > 1, 1.0, 0.0, keep_attrs=True)["a"].attrs)    # {}
print(xr.where(ds > 1, 1.0, 0.0, keep_attrs=False)["a"].attrs)   # {}

ones = xr.ones_like(ds)
print(xr.where(ds > 1, ones, 0.0, keep_attrs=True)["a"].attrs)   # {"foo": "bar", "baz": 1}
print(xr.where(ds > 1, ones, 0.0, keep_attrs=False)["a"].attrs)  # {}