fix: MultiSegmentFetcher 空 dict 崩溃 + BaseDataset assert 替换为显式 raise

- MultiSegmentFetcher.__len__: min([]) → 加空检查返回 0
- BaseDataset.get_index: assert 替换为 RuntimeError / ValueError
- BaseDataset.__len__: assert 替换为 early return 0

astrai/dataset/storage.py

@@ -188,6 +188,8 @@ class MultiSegmentFetcher:
 
     def __len__(self) -> int:
         """Returns the minimum length across all fetchers."""
+        if not self.multi_fetchers:
+            return 0
         len_list = [len(seg) for seg in self.multi_fetchers.values()]
         return min(len_list)