should `hydrate` coerce `None` to an empty dictionary or list?

[mishaschwartz]

The tests for pypgstac make it seem like the hydrate function is expected to be able to treat a None value as if it were an empty dictionary:

    def test_base_none(self) -> None:
        base_item = {"value": None}
        dehydrated = {"value": {"a": "b"}}
        hydrated = self.hydrate(base_item, dehydrated)
        assert hydrated == {"value": {"a": "b"}}

See the relevant code here: `https://github.com/stac-utils/pgstac/blob/6c569c0ee56bd9519ad947cbcaa8bfbcfade15bb/src/pypgstac/tests/hydration/test_hydrate.py#L241-L245

Right now, the hydrate function does not do that. Instead it tries to downcast the None value to a dict and raises an error since it can't do that:

hydraters/src/lib.rs

Lines 42 to 49 in d3946d3

	if let Ok(item) = item.downcast::<PyDict>() {
	if let Ok(base) = base.downcast::<PyDict>() {
	hydrate_dict(base, item)?;
	} else {
	return Err(PyValueError::new_err(
	"type mismatch: item is a dict, but the base was not",
	));
	}

Do we want to change the code here so that if item or base is None, we convert it to a dict or list? Or if not, we should update the tests in pgstac.

What do you think?

[gadomski]

Yup, I'll update — I must have pulled the test suite from somewhere else so I didn't get stac-utils/pgstac#263. Thanks for the catch.