fix mypy

py-polars/tests/unit/operations/namespaces/test_meta.py

@@ -149,6 +149,7 @@ def test_meta_tree_format(namespace_files_path: Path) -> None:
 def test_meta_show_graph(namespace_files_path: Path) -> None:
     e = (pl.col("foo") * pl.col("bar")).sum().over(pl.col("ham")) / 2
     dot = e.meta.show_graph(show=False, raw_output=True)
+    assert dot is not None
     assert len(dot) > 0
     # Don't check output contents since this creates a maintenance burden
     # Assume output check in test_meta_tree_format is enough