Add tests for NaT when performing dt.to_period (pandas-dev#24921)

Author: alexander-ponomaroff

pandas/tests/series/test_period.py

@@ -164,3 +164,12 @@ def test_end_time_timevalues(self, input_vals):
         result = s.dt.end_time
         expected = s.apply(lambda x: x.end_time)
         tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize('input_vals', [
+        ('2001'), ('NaT')
+    ])
+    def test_to_period(self, input_vals):
+        # GH 21205
+        expected = Series([input_vals], dtype='Period[D]')
+        result = Series([input_vals], dtype='datetime64[ns]').dt.to_period('D')
+        tm.assert_series_equal(result, expected)