Fix request hanging when synchronously returning a value in setErrorHandler

[weiliddat]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Comments

Resolves #3209

Make setErrorHandler consistent in handling return values, like in lib/handleRequest.js

fastify/lib/handleRequest.js

Lines 136 to 142 in 7db55d4

	if (result !== undefined) {
	if (result !== null && typeof result.then === 'function') {
	wrapThenable(result, reply)
	} else {
	reply.send(result)
	}
	}

[mcollina]

lgtm

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.