TST: Add test for groupby with datetime and NaT values (#61021)

Author: asharmalik19

pandas/tests/groupby/test_groupby.py

@@ -2978,3 +2978,21 @@ def test_groupby_multi_index_codes():
 
     index = df_grouped.index
     tm.assert_index_equal(index, MultiIndex.from_frame(index.to_frame()))
+
+
+def test_groupby_datetime_with_nat():
+    # GH##35202
+    df = DataFrame(
+        {
+            "a": [
+                to_datetime("2019-02-12"),
+                to_datetime("2019-02-12"),
+                to_datetime("2019-02-13"),
+                pd.NaT,
+            ],
+            "b": [1, 2, 3, 4],
+        }
+    )
+    grouped = df.groupby("a", dropna=False)
+    result = len(grouped)
+    assert result == 3