fix empty content deltas

kimi_k2_tool_parser.py

@@ -554,10 +554,12 @@ class KimiK2ToolParser(ToolParser):
 
         # 3. Return a delta if we have content or tool-call updates.
         if content or tool_call_deltas:
-            return DeltaMessage(
-                content=content,
-                tool_calls=tool_call_deltas if tool_call_deltas else None,
-            )
+            kwargs: dict[str, Any] = {}
+            if content:
+                kwargs["content"] = content
+            if tool_call_deltas:
+                kwargs["tool_calls"] = tool_call_deltas
+            return DeltaMessage(**kwargs)
 
         # Empty delta with token ids means EOS or closing tag — return
         # non-None so the serving framework can finalize finish_reason.