What happened?

I tried to assign values to a dataset with a selector-dict where a variable is missing the dim from the selector-dict. This raises a ValueError.

What did you expect to happen?

I expect that assigning works the same as selecting and it will ignore the missing dims.

Minimal Complete Verifiable Example

```Python import xarray as xr

ds = xr.Dataset({"a": ("x", [1, 2, 3]), "b": ("y", [4, 5])})

ds[{"x": 1}]

this works and returns:

<xarray.Dataset>

Dimensions: (y: 2)

Dimensions without coordinates: y

Data variables:

a int64 2

b (y) int64 4 5

ds[{"x": 1}] = 1

this fails and raises a ValueError

ValueError: Variable 'b': indexer {'x': 1} not available

```

Relevant log output

```Python Traceback (most recent call last): File "xarray/core/dataset.py", line 1591, in _setitem_check var_k = var[key] File "xarray/core/dataarray.py", line 740, in getitem return self.isel(indexers=self._item_key_to_dict(key)) File "xarray/core/dataarray.py", line 1204, in isel variable = self._variable.isel(indexers, missing_dims=missing_dims) File "xarray/core/variable.py", line 1181, in isel indexers = drop_dims_from_indexers(indexers, self.dims, missing_dims) File "xarray/core/utils.py", line 834, in drop_dims_from_indexers raise ValueError( ValueError: Dimensions {'x'} do not exist. Expected one or more of ('y',)

The above exception was the direct cause of the following exception:

Traceback (most recent call last): File "<stdin>", line 1, in <module> File "xarray/core/dataset.py", line 1521, in setitem value = self._setitem_check(key, value) File "xarray/core/dataset.py", line 1593, in _setitem_check raise ValueError( ValueError: Variable 'b': indexer {'x': 1} not available ```

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS

commit: None python: 3.9.1 (default, Jan 13 2021, 15:21:08) [GCC 4.8.5 20150623 (Red Hat 4.8.5-44)] python-bits: 64 OS: Linux OS-release: 3.10.0-1160.49.1.el7.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.12.0 libnetcdf: 4.7.4

xarray: 0.21.1 pandas: 1.4.0 numpy: 1.21.5 scipy: 1.7.3 netCDF4: 1.5.8 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.5.1.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: 3.5.1 cartopy: None seaborn: None numbagg: None fsspec: None cupy: None pint: None sparse: None setuptools: 49.2.1 pip: 22.0.3 conda: None pytest: 6.2.5 IPython: 8.0.0 sphinx: None