BUG: maybe_convert_objects with timedelta64 nat (#40062)

Author: jbrockmendel

pandas/_libs/lib.pyx

@@ -2250,7 +2250,7 @@ def maybe_convert_objects(ndarray[object] objects, bint try_float=False,
                 break
         elif is_timedelta(val):
             if convert_timedelta:
-                itimedeltas[i] = convert_to_timedelta64(val, 'ns')
+                itimedeltas[i] = convert_to_timedelta64(val, "ns").view("i8")
                 seen.timedelta_ = True
             else:
                 seen.object_ = True

pandas/tests/dtypes/test_inference.py

@@ -583,6 +583,16 @@ def test_maybe_convert_objects_datetime(self):
         )
         tm.assert_numpy_array_equal(out, exp)
 
+    def test_maybe_convert_objects_timedelta64_nat(self):
+        obj = np.timedelta64("NaT", "ns")
+        arr = np.array([obj], dtype=object)
+        assert arr[0] is obj
+
+        result = lib.maybe_convert_objects(arr, convert_timedelta=True)
+
+        expected = np.array([obj], dtype="m8[ns]")
+        tm.assert_numpy_array_equal(result, expected)
+
     @pytest.mark.parametrize(
         "exp",
         [