## Summary
- Fixes dead code elimination producing invalid syntax like `{ ...a, x:
}` when simplifying empty objects in spread contexts
- The issue was that `simplifyUnusedExpr` and `joinAllWithCommaCallback`
could return `E.Missing` instead of `null` to indicate "no side effects"
- Added checks to return `null` when the result is `E.Missing`
Fixes#25609
## Test plan
- [x] Added regression test that fails on v1.3.5 and passes with fix
- [x] `bun bd test test/regression/issue/25609.test.ts` passes
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Co-authored-by: Claude <noreply@anthropic.com>
