TST: added test for groupby when calling first, last, nth (GH29645) (#43358)

Author: lorenzophys

pandas/tests/groupby/test_nth.py

@@ -689,3 +689,20 @@ def test_first_multi_key_groupbby_categorical():
         [(1, 100), (1, 200), (2, 100)], names=["A", "B"]
     )
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize("method", ["first", "last", "nth"])
+def test_groupby_last_first_nth_with_none(method, nulls_fixture):
+    # GH29645
+    expected = Series(["y"])
+    data = Series(
+        [nulls_fixture, nulls_fixture, nulls_fixture, "y", nulls_fixture],
+        index=[0, 0, 0, 0, 0],
+    ).groupby(level=0)
+
+    if method == "nth":
+        result = getattr(data, method)(3)
+    else:
+        result = getattr(data, method)()
+
+    tm.assert_series_equal(result, expected)