Fix divide by zero error when all URI size estimations fail

- Add check for empty row_sizes list before calculating average
- Fall back to using number of rows in block when no file sizes are available
- Prevents ZeroDivisionError when all URIs fail to provide size information

Author: xyuzh

python/ray/data/_internal/planner/plan_download_op.py

@@ -281,6 +281,12 @@ def _estimate_nrows_per_partition(self, block: pa.Table) -> int:
         ]
 
         target_nbytes_per_partition = self._data_context.target_max_block_size
+        if len(row_sizes) == 0:
+            logger.warning(
+                "Unable to estimate file sizes for URIs. "
+                "Falling back to using the number of rows in the block as the partition size."
+            )
+            return len(block)
         avg_nbytes_per_row = sum(row_sizes) / len(row_sizes)
         if avg_nbytes_per_row == 0:
             logger.warning(