Improve code coverage by label_transfer tests

Checks if error is correctly raised when no KNN is present in adata.

tests/tools/_perturbation_space/test_simple_perturbation_space.py

@@ -240,9 +240,12 @@ def test_label_transfer():
     adata = AnnData(X)
     perturbations = np.array(["A", "B", "C"] * 22 + ["unknown"] * 3)
     adata.obs["perturbation"] = perturbations
-    sc.pp.neighbors(adata, use_rep="X")
-    sc.tl.umap(adata)
 
+    with pytest.raises(ValueError):
+        ps = pt.tl.PseudobulkSpace()
+        ps.label_transfer(adata)
+
+    sc.pp.neighbors(adata, use_rep="X")
     ps = pt.tl.PseudobulkSpace()
     ps.label_transfer(adata)
     assert "unknown" not in adata.obs["perturbation"]