fix: robust handling of empty result when reranking (#2313)

I found some edge cases while running experiments that - depending on
the base reranking libraries, some of them don't handle empty lists
well. This PR manually checks if the result set to be reranked is empty

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Enhanced search result processing by ensuring that reordering only
occurs when valid, non-empty results are available, thereby preventing
unnecessary operations and potential errors.

- **Tests**
- Added automated tests to verify that empty search result sets are
handled correctly, ensuring consistent behavior across various
rerankers.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

python/python/tests/test_rerankers.py

@@ -457,3 +457,45 @@ def test_voyageai_reranker(tmp_path, use_tantivy):
     reranker = VoyageAIReranker(model_name="rerank-2")
     table, schema = get_test_table(tmp_path, use_tantivy)
     _run_test_reranker(reranker, table, "single player experience", None, schema)
+
+
+def test_empty_result_reranker():
+    pytest.importorskip("sentence_transformers")
+    db = lancedb.connect("memory://")
+
+    # Define schema
+    schema = pa.schema(
+        [
+            ("id", pa.int64()),
+            ("text", pa.string()),
+            ("vector", pa.list_(pa.float32(), 128)),  # 128-dimensional vector
+        ]
+    )
+
+    # Create empty table with schema
+    empty_table = db.create_table("empty_table", schema=schema, mode="overwrite")
+    empty_table.create_fts_index("text", use_tantivy=False, replace=True)
+    for reranker in [
+        CrossEncoderReranker(),
+        # ColbertReranker(),
+        # AnswerdotaiRerankers(),
+        # OpenaiReranker(),
+        # JinaReranker(),
+        # VoyageAIReranker(model_name="rerank-2"),
+    ]:
+        results = (
+            empty_table.search(list(range(128)))
+            .limit(3)
+            .rerank(reranker, "query")
+            .to_arrow()
+        )
+        # check if empty set contains _relevance_score column
+        assert "_relevance_score" in results.column_names
+        assert len(results) == 0
+
+        results = (
+            empty_table.search("query", query_type="fts")
+            .limit(3)
+            .rerank(reranker)
+            .to_arrow()
+        )