[AURON #2482] handle optional sort indices for zero-column batches (#2483)

# Which issue does this PR close?

Closes #2482

# Rationale for this change

The primitive sort fast path changed `sorted_indices` from `Vec<u32>` to
`Option<Vec<u32>>`, but the zero-column batch path still calls `len()`
as if it were a `Vec`.

This causes `datafusion-ext-plans` to fail compilation.

# What changes are included in this PR?

Use the sorted indices length when present, and fall back to the number
of key rows for the primitive fast path.

# Are there any user-facing changes?

No.

# How was this patch tested?

UTs passed.

# Was this patch authored or co-authored using generative AI tooling?
- [x] Yes
- [ ] No

If yes, include: `Generated-by: <tool name and version>`

ASF guidance: https://www.apache.org/legal/generative-tooling.html

Co-authored-by: Shilun Fan <slfan1989@apache.org>
Signed-off-by: Shilun Fan <slfan1989@apache.org>

Author: lyne7-sc

native-engine/datafusion-ext-plans/src/sort_exec.rs

@@ -722,7 +722,7 @@ impl ExternalSorter {
                 take_batch(batch, sorted_indices.expect("non-fast-path indices"))?
             }
         } else {
-            create_zero_column_batch(sorted_indices.len())
+            create_zero_column_batch(sorted_indices.as_ref().map_or(keys.num_rows(), Vec::len))
         };
 
         // add to in-mem blocks