fix(python): graceful handling of empty result sets in hybrid search (#3030)

## Problem

When applying hard filters that result in zero matches, hybrid search
crashes with `IndexError: list index out of range` during reranking.
This happens because empty result tables are passed through the full
reranker pipeline, which expects at least one result.

Traceback from the issue:
```
lancedb/query.py: in _combine_hybrid_results
    results = reranker.rerank_hybrid(fts_query, vector_results, fts_results)
lancedb/rerankers/answerdotai.py: in rerank_hybrid
    combined_results = self._rerank(combined_results, query)
...
IndexError: list index out of range
```

## Fix

Added an early return in `_combine_hybrid_results` when both vector and
FTS results are empty. Instead of passing empty tables through
normalization, reranking, and score restoration (which can fail in
various ways), we now build a properly-typed empty result table with the
`_relevance_score` column and return it directly.

## Test

Added `test_empty_hybrid_result_reranker` that exercises
`_combine_hybrid_results` directly with empty vector and FTS tables,
verifying:
- Returns empty table with correct schema  
- Includes `_relevance_score` column
- Respects `with_row_ids` flag

Closes #2425

python/python/lancedb/query.py

@@ -1782,6 +1782,26 @@ class LanceHybridQueryBuilder(LanceQueryBuilder):
             vector_results = LanceHybridQueryBuilder._rank(vector_results, "_distance")
             fts_results = LanceHybridQueryBuilder._rank(fts_results, "_score")
 
+        # If both result sets are empty (e.g. after hard filtering),
+        # return early to avoid errors in reranking or score restoration.
+        if vector_results.num_rows == 0 and fts_results.num_rows == 0:
+            # Build a minimal empty table with the _relevance_score column
+            combined_schema = pa.unify_schemas(
+                [vector_results.schema, fts_results.schema],
+            )
+            empty = pa.table(
+                {
+                    col: pa.array([], type=combined_schema.field(col).type)
+                    for col in combined_schema.names
+                }
+            )
+            empty = empty.append_column(
+                "_relevance_score", pa.array([], type=pa.float32())
+            )
+            if not with_row_ids and "_rowid" in empty.column_names:
+                empty = empty.drop(["_rowid"])
+            return empty
+
         original_distances = None
         original_scores = None
         original_distance_row_ids = None

python/python/tests/test_rerankers.py

@@ -531,6 +531,78 @@ def test_empty_result_reranker():
         )
 
 
+def test_empty_hybrid_result_reranker():
+    """Test that hybrid search with empty results after filtering doesn't crash.
+
+    Regression test for https://github.com/lancedb/lancedb/issues/2425
+    """
+    from lancedb.query import LanceHybridQueryBuilder
+
+    # Simulate empty vector and FTS results with the expected schema
+    vector_schema = pa.schema(
+        [
+            ("text", pa.string()),
+            ("vector", pa.list_(pa.float32(), 4)),
+            ("_rowid", pa.uint64()),
+            ("_distance", pa.float32()),
+        ]
+    )
+    fts_schema = pa.schema(
+        [
+            ("text", pa.string()),
+            ("vector", pa.list_(pa.float32(), 4)),
+            ("_rowid", pa.uint64()),
+            ("_score", pa.float32()),
+        ]
+    )
+    empty_vector = pa.table(
+        {
+            "text": pa.array([], type=pa.string()),
+            "vector": pa.array([], type=pa.list_(pa.float32(), 4)),
+            "_rowid": pa.array([], type=pa.uint64()),
+            "_distance": pa.array([], type=pa.float32()),
+        },
+        schema=vector_schema,
+    )
+    empty_fts = pa.table(
+        {
+            "text": pa.array([], type=pa.string()),
+            "vector": pa.array([], type=pa.list_(pa.float32(), 4)),
+            "_rowid": pa.array([], type=pa.uint64()),
+            "_score": pa.array([], type=pa.float32()),
+        },
+        schema=fts_schema,
+    )
+
+    for reranker in [LinearCombinationReranker(), RRFReranker()]:
+        result = LanceHybridQueryBuilder._combine_hybrid_results(
+            fts_results=empty_fts,
+            vector_results=empty_vector,
+            norm="score",
+            fts_query="nonexistent query",
+            reranker=reranker,
+            limit=10,
+            with_row_ids=False,
+        )
+        assert len(result) == 0
+        assert "_relevance_score" in result.column_names
+        assert "_rowid" not in result.column_names
+
+    # Also test with with_row_ids=True
+    result = LanceHybridQueryBuilder._combine_hybrid_results(
+        fts_results=empty_fts,
+        vector_results=empty_vector,
+        norm="score",
+        fts_query="nonexistent query",
+        reranker=LinearCombinationReranker(),
+        limit=10,
+        with_row_ids=True,
+    )
+    assert len(result) == 0
+    assert "_relevance_score" in result.column_names
+    assert "_rowid" in result.column_names
+
+
 @pytest.mark.parametrize("use_tantivy", [True, False])
 def test_cross_encoder_reranker_return_all(tmp_path, use_tantivy):
     pytest.importorskip("sentence_transformers")