function validateTopics (topics) {
  if (topics.length === 0) {
    return 'empty_topic_list'
  }
  for (var i = 0; i < topics.length; i++) {
    if (!validateTopic(topics[i])) {
      return topics[i]
    }
  }
  return null
}
Validate an array of topics to see if any of them is valid or not
@param {Array} topics - Array of topics
@returns {String} If the topics is valid, returns null. Otherwise, returns the invalid one