[MINOR] Make composition of error and 'nil' return the original error

While working on adding missing error checks I noticed that calling err = errors.Compose(err, nil) won't result in err but instead [err]. That means we would need to search the codebase for hundreds of error check like err == ErrPeerExists and change them to errors.Contains(err, ErrPeerExists) since equality is broken by Compose. While possible, this is also risky cause missing a single check could be fatal. That's why I propose changing the problem at the root and adjust errors.Compose to return the error directly if only a single non-nil error was provided. This won't break errors.Contains but guarantee that we don't accidentally break error checks by doing things like

defer func() {
  err = errors.Compose(err, file.Close()) // results in [err] instead of err if file is closed successfully
}()