[Bugfix] Drop empty tool_calls lists to keep assistant replies in chat template (#30648)

Signed-off-by: Seokhyun An <iamseokhyun@gmail.com>

vllm/entrypoints/chat_utils.py

@@ -1629,12 +1629,17 @@ def _postprocess_messages(messages: list[ConversationMessage]) -> None:
     # so, for messages that have tool_calls, parse the string (which we get
     # from openAI format) to dict
     for message in messages:
-        if (
-            message["role"] == "assistant"
-            and "tool_calls" in message
-            and isinstance(message["tool_calls"], list)
-        ):
-            for item in message["tool_calls"]:
+        if message["role"] == "assistant" and "tool_calls" in message:
+            tool_calls = message.get("tool_calls")
+            if not isinstance(tool_calls, list):
+                continue
+
+            if len(tool_calls) == 0:
+                # Drop empty tool_calls to keep templates on the normal assistant path.
+                message.pop("tool_calls", None)
+                continue
+
+            for item in tool_calls:
                 # if arguments is None or empty string, set to {}
                 if content := item["function"].get("arguments"):
                     if not isinstance(content, (dict, list)):