[python] Allow obs_query=None/var_query=None in ExperimentAxisQuery

[johnkerl]

The default on var_query when it is unsupplied is
https://github.com/single-cell-data/SOMA/blob/python-v1.0.10/python-spec/src/somacore/query/query.py#L92
-- it is expecting axis.AxisQuery() as the 'unsupplied' value, rather than None

Users can avoid setting var_query=None, or, set var_query=tiledbsoma.AxisQuery()

But we should make it so that if users do pass var_query=None or obs_query=None, we can map that to AxisQuery() as the remaining code expects -- saying x=None is a common paradigm and we should let people say that.

Plan (FYI @ryan-williams ):

• PR Allow obs_query/var_query=None syntax SOMA#194 on the somacore repo
• Package release 1.0.11 on that repo
• PR on this repo to
  • Depend on bump of somacore
  • Add unit-test cases here

Repro script:

#!/usr/bin/env python

import tiledbsoma

soma_uri = "/whatever/path/to/pbmc3k"

obs_query = tiledbsoma.AxisQuery(value_filter='louvain == "B cells"')

with tiledbsoma.Experiment.open(soma_uri) as exp:
    query = tiledbsoma.ExperimentAxisQuery(
        experiment=exp,
        measurement_name="RNA",
        obs_query=obs_query,
        var_query=None, # setting this to None used to lead to an error
    )

query.to_anndata(
    "data",
    column_names=tiledbsoma.AxisColumnNames(obs=["soma_joinid", "obs_id", "louvain"], var=["soma_joinid", "var_id"]), 
    X_layers=()
)