[python] Allow adata.X to be None (#1767)

* [python] Ingest `adata.X==None`

* [python] Ingest `adata.X==None`

* neaten

* code-review feedback

apis/python/src/tiledbsoma/io/ingest.py

@@ -510,10 +510,10 @@ def from_anndata(
                 #   chunkwise into memory.
                 # Using the latter allows us to ingest larger .h5ad files without OOMing.
 
-                # Some AnnData objects have no X at all.
-                has_X = True
+                # Some AnnData objects have no X at all. This might be a missing attribute
+                # anndata.X; it might be anndata.X present, but with value None.
                 try:
-                    anndata.X
+                    has_X = anndata.X is not None
                 except (NameError, KeyError):
                     # We need to check both -- different exception types occur dependinng
                     # on whether the anndata object is read in backing mode or not.

apis/python/tests/test_basic_anndata_io.py

@@ -66,13 +66,18 @@ def h5ad_file_categorical_int_nan(request):
 
 @pytest.fixture
 def h5ad_file_X_empty(request):
-    input_path = HERE.parent / "testdata/xempty.h5ad"
+    """adata.X is a zero-cell sparse matrix"""
+    input_path = HERE.parent / "testdata/x-empty.h5ad"
     return input_path
 
 
 @pytest.fixture
 def h5ad_file_X_none(request):
-    input_path = HERE.parent / "testdata/xnone.h5ad"
+    """
+    adata.X has Python value None if read in non-backed mode; if read in backed
+    mode, adata.X is not present as an attribute of adata.
+    """
+    input_path = HERE.parent / "testdata/x-none.h5ad"
     return input_path