fix: use isnat

Author: ilan-gold

src/zarr/core/dtype/npy/time.py

@@ -545,7 +545,7 @@ def cast_scalar(self, data: object) -> np.timedelta64:
         raise a TypeError.
         """
         if self._check_scalar(data):
-            if isinstance(data, np.timedelta64) and np.isnan(data):
+            if isinstance(data, np.timedelta64) and np.isnat(data):
                 return np.timedelta64("NaT", self.unit)
             return self._cast_scalar_unchecked(data)
         msg = (