Empty query results typing (#101)

* change default empty matrix dtype in fast csr builder

* ensure correct dtype on empty axis dataframe

* set correct shape on empty result

python-spec/src/somacore/query/_fast_csr.py

@@ -117,13 +117,15 @@ def finalize(self) -> _CSRAccumulatorFinalResult:
         nnz = sum(len(chunk[2]) for chunk in self.coo_chunks)
         index_dtype = _select_dtype(nnz)
         if nnz == 0:
-            # no way to infer matrix dtype, so use default and return empty matrix
-            empty = sparse.csr_matrix((0, 0))
+            # There is no way to infer matrix dtype, so use a default and return
+            # an empty matrix. Float32 is used as a default type, as it is most
+            # compatible with AnnData expectations.
+            empty = sparse.csr_matrix((0, 0), dtype=np.float32)
             return _CSRAccumulatorFinalResult(
                 data=empty.data,
                 indptr=empty.indptr,
                 indices=empty.indices,
-                shape=(0, 0),
+                shape=self.shape,
             )
 
         # cumsum row lengths to get indptr

python-spec/src/somacore/query/query.py

@@ -398,10 +398,10 @@ class _AxisQueryResult:
     def to_anndata(self) -> anndata.AnnData:
         """Convert to AnnData"""
         obs = self.obs.to_pandas()
-        obs.index = obs.index.map(str)
+        obs.index = obs.index.astype(str)
 
         var = self.var.to_pandas()
-        var.index = var.index.map(str)
+        var.index = var.index.astype(str)
 
         return anndata.AnnData(
             X=self.X, obs=obs, var=var, layers=(self.X_layers or None)