refactor: address review feedback

- Use find_map(|p| p.first()) instead of flatten().next() to make the
  "first batch in any partition" intent clearer (per @kosiew).
- Extend regression test to also assert the empty outer partition list
  case ([]) raises the same ValueError.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: fangchenli

crates/core/src/context.rs

@@ -837,8 +837,7 @@ impl PySessionContext {
         let schema = partitions
             .0
             .iter()
-            .flatten()
-            .next()
+            .find_map(|partition| partition.first())
             .ok_or_else(|| {
                 PyValueError::new_err(
                     "Cannot register record batches without a schema: the \

python/tests/test_context.py

@@ -122,6 +122,10 @@ def test_register_record_batches_empty(ctx):
     with pytest.raises(ValueError, match="no record batches"):
         ctx.register_record_batches("t", [[]])
 
+    # An empty outer partition list carries no schema either, and raises the same error.
+    with pytest.raises(ValueError, match="no record batches"):
+        ctx.register_record_batches("t", [])
+
     # The schema is still recovered from a later non-empty partition.
     batch = pa.RecordBatch.from_arrays([pa.array([1, 2, 3])], names=["a"])
     ctx.register_record_batches("t2", [[], [batch]])