!2231 fix report error in get_dataset_size

Merge pull request !2231 from yanghaitao/yht_get_num_samples

mindspore/dataset/engine/samplers.py

@@ -115,6 +115,8 @@ class Sampler:
         return self.child_sampler.is_sharded()
 
     def get_num_samples(self):
+        if self.num_samples is None:
+            return None
         return self._get_indices().size