fix bug when fileSource variable is empty

[wuzhiming]

Re: #

Changelog

• Before reading fileSource, it is necessary to check both whether the fileSource field exists and whether the file itself exists.（found it from sentry ）

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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.

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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 is not changed

Interface Check Report

This pull request does not change any public interfaces !