fix pydantic dependency

[Netrvin]

PR types

Bug fixes

PR changes

One of dependencies (pydantic)

Describe

When pydantic>=2.0, str cannot be directly None, it must be specified using Optional[str] or similar methods. If pydantic>=2.0 is used, it will lead to issues like the following:
In paddlespeech/server/restful/response.py, the TTSResponse's save_path: str = None will raise an error when save_path is None, thus causing an ErrorResponse to be returned.

[CLAassistant]

All committers have signed the CLA.

[zxcd]

LGTM

[luotao1]

hi, @Netrvin

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