function getNativeUserAgentString() {
  const navigator = getNavigator();
  if (navigator) {
    const userAgent = navigator.userAgent;
    if (userAgent) {
      return userAgent;
    }
  }
  return '';
}
Gets the native userAgent string from navigator if it exists.
If navigator or navigator.userAgent string is missing, returns an empty
string.
@return {string}