fix: unclosed <think> tag when content is empty

[xiaokunzh]

• fix unclosed tag when content is empty (tool_calls or others)
• see the same reported issue Endless thinking. Agent with llm which opened enable_thinking and using a tool will lost </think> tag dify#25482

[Yeuoly]

Hi, regarding the absence of test cases for this feature, I'm uncertain whether these changes could cause any issues. Would you mind adding some test cases to address this?