Add testing for NaT on datetime values

Author: moradology

tests/test_properties.py

@@ -287,7 +287,9 @@ def test_array_metadata_meets_spec(meta: ArrayV2Metadata | ArrayV3Metadata) -> N
     # version-agnostic validations
     if meta.dtype.kind == "f":
         assert serialized_float_is_valid(asdict_dict["fill_value"])
-    if meta.dtype.kind == "c":
+    elif meta.dtype.kind == "c":
         # fill_value should be a two-element array [real, imag].
         assert serialized_float_is_valid(asdict_dict["fill_value"].real)
         assert serialized_float_is_valid(asdict_dict["fill_value"].imag)
+    elif meta.dtype.kind == "M" and np.isnat(meta.fill_value):
+        assert asdict_dict["fill_value"] == "NaT"