Visium HD: empty dataset_id default produces an unnamed coordinate system that napari-spatialdata can't select #422

Hi, and thanks for maintaining spatialdata-io!

When the Visium HD files have no filename prefix (e.g. feature_slice.h5) and dataset_id is not provided, visium_hd() falls back to dataset_id="".

Since dataset_id is used both as element prefix and as coordinate-system name, this yields an empty coordinate system that is problematic downstream: for instance, napari-spatialdata silently ignores it and shows it as a blank row, so the full-resolution image can't be displayed at all. This was reported as a Sopa bug in prism-oncology/sopa#446 before we traced it here.

That could be avoided by either defaulting to a non-empty identifier, or keeping "" but naming the coordinate system "global" when dataset_id is empty.

Thanks!