fix(rerankers): guard against empty vector_results in RRFReranker.rerank_multivector (#3467)

## What's broken

Calling `RRFReranker().rerank_multivector([])` crashes with `IndexError:
list index out of range` because the method accesses `vector_results[0]`
for the type-homogeneity check before verifying the list is non-empty.
The `all()` call passes vacuously on an empty iterable so the crash hits
the next lines.

```python
from lancedb.rerankers import RRFReranker
RRFReranker().rerank_multivector([])
# IndexError: list index out of range
```

## Why it happens

The type check uses `vector_results[0]` as the reference type but never
guards against an empty list. `all(...)` short-circuits to `True` when
the iterable is empty, so the bad index access on the lines that follow
is never reached by the existing guard logic.

## Fix

Add an explicit empty-list check before any indexing.

python/python/lancedb/rerankers/rrf.py

@@ -82,6 +82,9 @@ class RRFReranker(Reranker):
         results from multiple vector searches as it doesn't support reranking
         vector results individually.
         """
+        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):
             raise ValueError(