[v3.8.5] Fix constructing spine::String with null pointer.

[dumganhar]

It's a bug from v3.8.5-beta0. ( #17783 )

Issue: strlen(nullptr) will cause crash.

Re: https://forum.cocos.org/t/topic/163455/243?u=dumganhar

Changelog

---

Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

---

Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[github-actions]

⚠️ Package size ⤴ 78 bytes, old: 5440959, new: 5441037

Interface Check Report

This pull request does not change any public interfaces !