perf: Set content to None when the OpenAI message content list is empty (#6551)

_finally_convert_payload 提取 think 部分后，如果 assistant 消息的
所有 content 都是 think 类型，new_content 会变成空列表 []。
Grok 等 provider 不接受空 content list，直接报 400。

改为 new_content or None，空列表时回退到 None（OpenAI 兼容 API
普遍接受 null content 的 assistant 消息）。

Fixes #6447

Co-authored-by: Yufeng He <40085740+universeplayer@users.noreply.github.com>

Author: he-yufeng

astrbot/core/provider/sources/openai_source.py

@@ -877,8 +877,9 @@ def _finally_convert_payload(self, payloads: dict) -> None:
                         reasoning_content += str(part.get("think"))
                     else:
                         new_content.append(part)
-                message["content"] = new_content
-                # reasoning key is "reasoning_content"
+                # Some providers (Grok, etc.) reject empty content lists.
+                # When all parts were think blocks, fall back to None.
+                message["content"] = new_content or None
                 if reasoning_content:
                     message["reasoning_content"] = reasoning_content