Support empty string messages

[domarp-j]

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

This PR proposes supporting empty strings as messages to support the following use case:

const MyError = createError("MY_CODE", "", 400)
// before -> throws error
// after -> works

throw new MyError() // message = ""
throw new MyError("broken") // message = "broken"

This supports the use case where the user doesn't desire any prefix or suffix to the message; they want the message to match the argument passed to the error.

The trim() I admit is a little problematic, but without it, the above example looks like:

throw new MyError("broken") // message = " broken" (note the space)

[climba03003]

Not sure which one will be better, most of the time the if clause will be executed.

Suggested change

	this.message = format(message, ...args).trim()
	if (message) args.unshift(message)
	this.message = format(...args)

[jsumners]

I would:

1. if (typeof message !== 'string') throw Error('Fastify error message must be defined') (line 11)
2. if (message === '') { this.message = message } else { this.message = format(message, ...args) }

[domarp-j]

@climba03003 I implemented your solution; thank you.

@jsumners I tried your solution, but unfortunately it didn't pass tests.

[Uzlopak]

I dislike the solution.

[domarp-j]

@Uzlopak I appreciate the useful feedback. Closing and implementing my own solution that addresses our particular use case. Thank you for your assistance.