fix all_none feather test for non-string dtype

Author: jorisvandenbossche

python/pyarrow/tests/test_feather.py

@@ -426,7 +426,7 @@ def test_empty_strings(version):
 @pytest.mark.pandas
 def test_all_none(version):
     df = pd.DataFrame({'all_none': [None] * 10})
-    if version == 1:
+    if version == 1 and pa.pandas_compat._pandas_api.uses_string_dtype():
         expected = df.astype("str")
     else:
         expected = df