fix: 修复chroma_store.py中unsupported-operand-type问题

[china-zhz]

Description

在使用Chroma vector store没有指定ChromaVectorConfig中的persist_path属性时，项目启动会报错如下：

其中chroma_vector_config.get("persist_path", os.path.join(PILOT_PATH, "data")) 执行结果是None而不是os.path.join(PILOT_PATH, "data")的值，因为字典类型的get方法只有当不包含此键时才会使用指定的默认值，但是chroma_vector_config字典中persist_path键值存在只不过值为None

How Has This Been Tested?

使用Chroma vector store ->新增一个数据库 -> 项目启动 问题可复现，测试流程也如此

Checklist:

• [√] My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• [√] I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules