function getServerName (host) {
  if (!host) {
    return null
  }

  assert(typeof host === 'string')

  const servername = getHostname(host)
  if (net.isIP(servername)) {
    return ''
  }

  return servername
}
IP addresses are not valid server names per RFC6066
Currently, the only server names supported are DNS hostnames
@param {string|null} host
@returns {string|null}