fix(data): normalize null chat content

Signed-off-by: yaoyu-33 <yaoyu.094@gmail.com>

Author: yaoyu-33

src/megatron/bridge/data/conversation_processing.py

@@ -578,7 +578,8 @@ def normalize_chat_conversation(
     """Normalize supported text-chat schemas to OpenAI-style messages.
 
     Rows may use ``messages``, singular ``conversation``, or legacy plural
-    ``conversations`` with ``from``/``value`` keys.
+    ``conversations`` with ``from``/``value`` keys. Null OpenAI message content
+    is converted to an empty string before chat-template rendering.
     """
     if isinstance(example_or_conversation, Mapping):
         source = example_or_conversation
@@ -603,6 +604,8 @@ def normalize_chat_conversation(
             raise ValueError("Chat conversation turns must be dictionaries.")
         turn = dict(raw_turn)
         if "role" in turn and "content" in turn:
+            if turn["content"] is None:
+                turn["content"] = ""
             _normalize_assistant_tool_call_arguments(turn)
             normalized.append(turn)
             continue

tests/unit_tests/data/test_conversation_processing.py

@@ -1339,7 +1339,7 @@ def test_shared_chat_preprocessing_normalizes_sharegpt_roles_before_templating()
     assert tokenized.assistant_mask.any()
 
 
-def test_normalize_chat_conversation_parses_openai_tool_call_arguments_without_mutating_input():
+def test_normalize_chat_conversation_normalizes_openai_tool_calls_without_mutating_input():
     row = {
         "messages": [
             {"role": "user", "content": "Weather?"},
@@ -1359,7 +1359,9 @@ def test_normalize_chat_conversation_parses_openai_tool_call_arguments_without_m
 
     normalized = normalize_chat_conversation(row)
 
+    assert normalized[1]["content"] == ""
     assert normalized[1]["tool_calls"][0]["function"]["arguments"] == {"city": "Seattle"}
+    assert row["messages"][1]["content"] is None
     assert row["messages"][1]["tool_calls"][0]["function"]["arguments"] == '{"city":"Seattle"}'
 
 

tests/unit_tests/models/test_glm52_kimi_k3_minimax_m3_chat.py

@@ -157,7 +157,7 @@ def _messages(*, tools: bool, thinking: bool) -> list[dict[str, Any]]:
             {
                 "role": "assistant",
                 "reasoning_content": "reason one" if thinking else None,
-                "content": "",
+                "content": None,
                 "tool_calls": [{"function": {"name": "weather", "arguments": '{"city":"Seattle"}'}}],
             },
             {"role": "tool", "content": "12 C"},