Allow empty coords?

When trying to use `az.from_dict` I would like to be able to specify dims without coords.

For example say I have a variable `x`, and a deterministic `x[::-1]`, they would have the same dims but reversed coords. It's currently not possible to specify coords per variable using the `from_dict` constructor, which I think it's fine, at that point you may as well use xarray directly.

But could we perhaps allow unspecified coords for some dims?

```python
import arviz as az
import numpy as np

az.from_dict({"x": np.zeros((1, 1, 5))}, dims={"x": ["a"]}, coords={"a": None})
```

If it's too much of an ask I'll go down the venue of creating a dataset.

Start at the az.from_dict entry point and trace how dims and coords are processed. Use the example in the issue to verify that coords={"a": None} permits a dimension without supplied coordinates, while preserving the requested dimension mapping.

Written by the indexing model from the issue text.