fix: handle null stats JSON in Socket server

[chenjiahan]

Summary

Handle null stats JSON in Socket server.

If the previous compilation is removed on the Rust side, stats.toJson gets a null instead of an object.

Failed to get stats due to error: Unable to access compilation with id = CompilationId(1) now. The compilation have been removed on the Rust side. The latest compilation id is CompilationId(4), are you trying to access the stats from the previous compilation?
file:///user/app/node_modules/.pnpm/@rsbuild+core@1.2.18/node_modules/@rsbuild/core/dist/index.js:5275
        if (statsJson.entrypoints) for (let entrypoint of Object.values(statsJson.entrypoints)){
                      ^

TypeError: Cannot read properties of null (reading 'entrypoints')
    at SocketServer.sendStats 

Related Links

https://discord.com/channels/977448667919286283/1167025132905173063/1349740225999405218
web-infra-dev/rspack#9687

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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