TST: Added test for mapping on mixed NA (#20574)

Closes #20495

Author: readyready15728

pandas/tests/series/test_apply.py

@@ -576,3 +576,14 @@ def f(x):
         result = s.map(f)
         exp = pd.Series(['Asia/Tokyo'] * 25, name='XX')
         tm.assert_series_equal(result, exp)
+
+    @pytest.mark.parametrize("vals,mapping,exp", [
+        (list('abc'), {np.nan: 'not NaN'}, [np.nan] * 3 + ['not NaN']),
+        (list('abc'), {'a': 'a letter'}, ['a letter'] + [np.nan] * 3),
+        (list(range(3)), {0: 42}, [42] + [np.nan] * 3)])
+    def test_map_missing_mixed(self, vals, mapping, exp):
+        # GH20495
+        s = pd.Series(vals + [np.nan])
+        result = s.map(mapping)
+
+        tm.assert_series_equal(result, pd.Series(exp))