[Bugfix] Properly reject requests with empty list guided_choice (#20195)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/v1/engine/processor.py

@@ -173,6 +173,12 @@ class Processor:
             params.guided_decoding.backend = engine_level_backend
 
         # Request content validation
+        if (isinstance(params.guided_decoding.choice, list)
+                and not params.guided_decoding.choice):
+            # It is invalid for choice to be an empty list
+            raise ValueError(f"Choice '{params.guided_decoding.choice}' "
+                             "cannot be an empty list")
+
         if engine_level_backend.startswith("xgrammar"):
             # xgrammar with no fallback
             validate_xgrammar_grammar(params)