fix: Fix the generate_prompt method generated a 'None' string

[wangliang181230]

What this PR does / why we need it?

fix: Fix the generate_prompt method generated a 'None' string

修复方法 generate_prompt 生成了 'None' 字符串的问题。
注：调用端并不是生成提示词时，这是错误的，应该返回None，而不是 'None' 字符串。

Summary of your change

用 判断器 做测试：

1. 修复前：（a=None，b=None，但 a != b，因为 None != 'None'）
   

2. 修复后：（a=None，b=None，a == b， None == None）
   

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.