function normalizeUrl(rootURL) {
  if (rootURL === undefined || rootURL === null || rootURL === '') {
    return '';
  }

  return cleanBaseURL(rootURL);
}
Returns a normalized url given a string.
Returns an empty string if `null`, `undefined` or an empty string are passed
in.

@method normalizeUrl
@param {String} Raw url.
@return {String} Normalized url.