[python] Support X-free AnnData files

[johnkerl]

This is from user feedback.

Update 2023-10-09:

When one writes an AnnData object with X=None, then on readback, two different possibilities occur on read-in:

• If adata = anndata.read_h5ad('the.h5ad', 'r'), then adata.X does not exist as an attribute of adata
• If adata = anndata.read_h5ad('the.h5ad'), then adata.X has Python value None

#1672 covered the former. #1767 covers the latter.

[johnkerl]

#1672 is merged and released in 1.5.0rc1.

[johnkerl]

[sc-35075]