fix: set the correct asset_info when extracting css

[inottn]

Summary

fix #8431

The default asset_info is returned here: https://github.com/web-infra-dev/rspack/blob/9112e35f97eeac430691e15c0305763fec56e890/crates/rspack_plugin_extract_css/src/plugin.rs#L415

Therefore, the contenthash cannot be obtained here, leading to an unexpected termination of the subsequent process: https://github.com/web-infra-dev/rspack/blob/9112e35f97eeac430691e15c0305763fec56e890/crates/rspack_plugin_real_content_hash/src/lib.rs#L73-L75

Checklist

• [x] Tests updated (or not required).
• [x] Documentation updated (or not required).

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[JSerFeng]

Thanks !