Fix last empty content delta in Case 5 (post-section-close)

Return None instead of DeltaMessage(content='') when delta_text
exists but there's no new content after the section end marker.

kimi_k2_tool_parser.py

@@ -554,6 +554,6 @@ class KimiK2ToolParser(ToolParser):
                     if new_after:
                         return DeltaMessage(content=new_after)
                     break
-            return DeltaMessage(content="") if delta_text else None
+            return None
 
         return None