TST: Add test for grouby after dropna agg nunique and unique (pandas-dev#57711)

* Add test

* typo

Author: Zhengbo Wang

pandas/tests/groupby/test_groupby.py

@@ -2945,3 +2945,23 @@ def test_decimal_na_sort(test_series):
     result = gb._grouper.result_index
     expected = Index([Decimal(1), None], name="key")
     tm.assert_index_equal(result, expected)
+
+
+def test_groupby_dropna_with_nunique_unique():
+    # GH#42016
+    df = [[1, 1, 1, "A"], [1, None, 1, "A"], [1, None, 2, "A"], [1, None, 3, "A"]]
+    df_dropna = DataFrame(df, columns=["a", "b", "c", "partner"])
+    result = df_dropna.groupby(["a", "b", "c"], dropna=False).agg(
+        {"partner": ["nunique", "unique"]}
+    )
+
+    index = MultiIndex.from_tuples(
+        [(1, 1.0, 1), (1, np.nan, 1), (1, np.nan, 2), (1, np.nan, 3)],
+        names=["a", "b", "c"],
+    )
+    columns = MultiIndex.from_tuples([("partner", "nunique"), ("partner", "unique")])
+    expected = DataFrame(
+        [(1, ["A"]), (1, ["A"]), (1, ["A"]), (1, ["A"])], index=index, columns=columns
+    )
+
+    tm.assert_frame_equal(result, expected)