function valueFromBundle(bundle, errors, message, args) {
  if (message.value) {
    return bundle.formatPattern(message.value, args, errors);
  }

  return null;
}
Format the value of a message into a string or `null`.

This function is passed as a method to `keysFromBundle` and resolve
a value of a single L10n Entity using provided `FluentBundle`.

If the message doesn't have a value, return `null`.

@param   {FluentBundle} bundle
@param   {Array<Error>} errors
@param   {Object} message
@param   {Object} args
@returns {string|null}
@private