fix(rerankers/mrr): raise ValueError on empty vector_results list (#3469)

## What's broken

`MRRReranker.rerank_multivector([])` raises `IndexError: list index out
of range`. The crash happens on line 128 (the `all()` type-homogeneity
check passes vacuously on an empty iterable) and on line 134 which
accesses `vector_results[0]` unconditionally, with no prior guard for an
empty list.

## Why it happens

`all()` over an empty iterable returns `True`, so the type check
silently passes and execution falls through to `vector_results[0]` which
crashes.

## Fix

Added a two-line guard at the top of `rerank_multivector` that raises a
clear `ValueError("vector_results must not be empty")` before any
indexing occurs.

## Test

Added `test_mrr_reranker_empty_input` in `test_rerankers.py` which calls
`rerank_multivector([])` and asserts that a `ValueError` with the
message "must not be empty" is raised.

Fixes #3468

Co-authored-by: Aegis Dev <aegis@devteamaegis.com>

python/python/lancedb/rerankers/mrr.py

@@ -125,6 +125,9 @@ class MRRReranker(Reranker):
         This cannot reuse rerank_hybrid because MRR semantics require treating
         each vector result as a separate ranking system.
         """
+        if not vector_results:
+            raise ValueError("vector_results must not be empty")
+
         if not all(isinstance(v, type(vector_results[0])) for v in vector_results):
             raise ValueError(
                 "All elements in vector_results should be of the same type"

python/python/tests/test_rerankers.py

@@ -344,6 +344,12 @@ def test_mrr_reranker(tmp_path):
     assert len(result_deduped) == len(result)
 
 
+def test_mrr_reranker_empty_input():
+    reranker = MRRReranker()
+    with pytest.raises(ValueError, match="must not be empty"):
+        reranker.rerank_multivector([])
+
+
 def test_rrf_reranker_distance():
     data = pa.table(
         {