fix(graph): filter ChatResponse.getResult() is null event

[wuzkcool]

Describe what this PR does / why we need it

使用graph时出现了NPE。 错误详情： java.lang.NullPointerException: Cannot invoke "org.springframework.ai.chat.model.Generation.getOutput()" because the return value of "org.springframework.ai.chat.model.ChatResponse.getResult()" is null

已经有多个提交试图修复这个问题，他们添加了非空判断，但是没有完全覆盖所有的逻辑分支。比如：fix: result empty (#2464)

Does this pull request fix one issue?

Describe how you did it

在org.springframework.ai.chat.model.ChatResponse的方法定义，getResult()方法是会存在返回为null的场景。在spring ai alibaba中应该正确的处理返回值为null的场景
```
	public Generation getResult() {
	if (CollectionUtils.isEmpty(this.generations)) {
		return null;
	}
	return this.generations.get(0);
}
```

Describe how to verify it

使用graph时，开开启流式响应，使用qwen-max 模型当前ChatResponse.getResult()=null的某个chunk 时会出现npe

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