fix(python): reject empty multivector reranker input

Reject empty multivector result lists with `ValueError` before accessing the first result, matching the existing RRF and MRR behavior. Non-empty lists, zero-row tables, and both deduplication modes retain their existing behavior.

Regression coverage exercises the shared base reranker through both deduplication settings. The exact-head Python suite passed with 3021 passed, 126 skipped, and 70 deselected.

python/python/lancedb/rerankers/base.py

@@ -189,6 +189,8 @@ class Reranker(ABC):
         vector_results = (
             [vector_results] if not isinstance(vector_results, list) else vector_results
         )
+        if not vector_results:
+            raise ValueError("vector_results must not be empty")
 
         # Make sure all elements are of the same type
         if not all(isinstance(v, type(vector_results[0])) for v in vector_results):

python/python/tests/test_rerankers.py

@@ -28,6 +28,7 @@ from lancedb.rerankers import (
     WatsonxReranker,
 )
 from lancedb.table import LanceTable
+from lancedb.rerankers.base import Reranker
 
 
 def get_test_table(tmp_path):
@@ -358,6 +359,19 @@ def test_mrr_reranker(tmp_path):
     assert len(result_deduped) == len(result)
 
 
+@pytest.mark.parametrize("deduplicate", [False, True])
+def test_base_reranker_multivector_empty_input(deduplicate):
+    class VectorReranker(Reranker):
+        def rerank_hybrid(self, query, vector_results, fts_results):
+            raise AssertionError("should not rerank")
+
+        def rerank_vector(self, query, vector_results):
+            raise AssertionError("should not rerank")
+
+    with pytest.raises(ValueError, match="vector_results must not be empty"):
+        VectorReranker().rerank_multivector([], None, deduplicate=deduplicate)
+
+
 def test_mrr_reranker_empty_input():
     reranker = MRRReranker()
     with pytest.raises(ValueError, match="must not be empty"):