fix(api/task): guard empty results list in process_results

For generate_until / generate_visual_cot tasks, when a sample's
generation failed upstream (model raised, retries exhausted, etc.)
the results list can be empty. The existing isinstance check then
falls through to `results[0]` on the list-of-list branch and raises
IndexError — which aborts the full postprocess loop for that task,
not just the missing sample.

Add a leading `results and` so an empty list falls through to the
else branch that does `[res.strip() for res in results]` (empty
output) — downstream task-level process_results then receives an
empty list and can decide how to score the missing sample without
taking down the whole task.

Author: Luodian

lmms_eval/api/task.py

@@ -1576,7 +1576,10 @@ def construct_requests(self, doc_id: int, ctx: str, **kwargs) -> Union[List[Inst
     @retry(stop=(stop_after_attempt(5) | stop_after_delay(1200)), wait=wait_fixed(2))
     def process_results(self, doc, results, full_docs=None):
         if self.OUTPUT_TYPE in ("generate_until", "generate_visual_cot"):
-            if isinstance(results, list) and isinstance(results[0], list):
+            # Guard empty results so results[0] below does not IndexError for
+            # samples whose generation failed. Downstream process_results then
+            # receives an empty list and can decide how to score the miss.
+            if results and isinstance(results, list) and isinstance(results[0], list):
                 results = [res.strip() for res in results[0]]
             else:
                 results = [res.strip() for res in results]