styleq: <cssAtom> typeof undefined is not "string" or "null".

Describe the issue

I recently updated from RSD 0.33 to 0.55 and started encountering this error.

Returning undefined for a property in an inline style function now causes this error. I believe these values were previously ignored / stripped by RSD.

Expected behavior

If this is an intentional change, I would expect the typescript types for this function to disallow undefined values.

Steps to reproduce

const styles = css.create({
  gap: (gap?: number) => ({
    rowGap: gap, // An undefined gap will result in styleq: rowGap-abcdef typeof undefined is not "string" or "null".
  }),
})

Test case

No response

Additional comments

No response

Start by reproducing the example using css.create and an undefined gap value, then trace the inline style handling that produces the styleq error. Check the TypeScript types for inline style functions and existing tests around undefined style values. Done means the reported case has defined behavior and the relevant test or types reflect it.