Check explicitly for empty cell when handling spglib 2.4 TypeError

The "positions has to be" string can also arise from other
inconsistencies in data size, which we might be less happy about.

euphonic/util.py

@@ -570,7 +570,10 @@ def _recip_space_labels(qpts: np.ndarray,
             sym_label_to_coords = seekpath.get_path(cell)["point_coords"]
         except (SymmetryDetectionError, TypeError) as err:
             if isinstance(err, TypeError):
+                # There is a particular TypeError we expect to see when the
+                # unit cell is empty; make sure we do not have some other error
                 assert "positions has to be" in str(err)
+                assert len(cell[1]) == 0
             warnings.warn(('Could not determine cell symmetry, using generic '
                            'q-point labels'), stacklevel=2)
             sym_label_to_coords = _generic_qpt_labels()