adding comment

Author: rcap107

skrub/tests/test_on_subframe.py

@@ -71,8 +71,12 @@ def test_empty_output(df_module, use_fit_transform):
         out = transformer.fit_transform(df)
     else:
         out = transformer.fit(df).transform(df)
+    # Selecting no columns to have an empty dataframe
     selected = s.select(df, ())
 
+    # I need to add a special case for pandas 3.0 here because the type of the
+    # empty dataframe with pandas 3.0 is different from that of out, but here
+    # we don't care about that dtype.
     if sbd.is_pandas(df) and parse(pd.__version__).major >= parse("3.0.0").major:
         out = sbd.to_numpy(out)
         selected = sbd.to_numpy(selected)