Fix return value in esbuild plugin `onLoad` hook

[nedjulius]

What changed / motivation ?

This PR fixes the return value of onLoad hook of the StyleX esbuild plugin (of the if branch when babel transform returns no code). Previously, it would return an object with property code, which resulted in undefined output of the plugin.

Linked PR/Issues

I did not create an issue for this since it's a very small fix.

Additional Context

Screenshots, Tests, Anything Else

Pre-flight checklist

• [x] I have read the contributing guidelines Contribution Guidelines
• [x] Performed a self-review of my code

[nmn]

Thanks! Will publish a release within a couple of days with this fix.