Load Visium HD generated without microscopy image

Hi,

SpaceRanger can be run on Visium HD data without a full resolution microscopy image, and only the CytAssist image.
This will create a metadata_json with "microscope_colrow_to_spot_colrow": null, which will then fail when reading with spatialdata_io.visium_hd:

https://github.com/scverse/spatialdata-io/blob/625c77c4ff371e3d23e44aac0706abbaa2cf705a/src/spatialdata_io/readers/visium_hd.py#L459

Traceback (most recent call last):
  File "read_data.py", line 57, in <module>
    spatialdata = spatialdata_io.visium_hd(
  File "spatialdata_io/readers/visium_hd.py", line 110, in visium_hd
    transform_matrices = _get_transform_matrices(metadata, hd_layout)
  File "spatialdata_io/readers/visium_hd.py", line 426, in _get_transform_matrices
    transform_matrices[key.value] = _get_affine(data)
  File "spatialdata_io/readers/visium_hd.py", line 398, in _get_affine
    matrix = np.array(coefficients).reshape(3, 3)
ValueError: cannot reshape array of size 1 into shape (3,3)

I am trying to build a subsample test dataset and would like to run without the microscopy image to win some space. Do you think it is possible to read without it?

Thanks!

Start in src/spatialdata_io/readers/visium_hd.py at _get_transform_matrices and _get_affine, using the reported metadata_json case where microscope_colrow_to_spot_colrow is null. Reproduce the reshape failure with a Visium HD dataset containing only the CytAssist image; done means spatialdata_io.visium_hd can read that dataset without a full-resolution microscopy image.