message: basket.js: Modify undefined --> null
  When the user opens a new tab, the session storage doesn't
  contain the "bid" anymore. As a result of which the value is
  marked as "null". This causes the challenge to solve accidentaly.
  Closes
diff: @@ -11,7 +11,7 @@ module.exports = function retrieveBasket () {
  /* jshint eqeqeq:false */
  if (utils.notSolved(challenges.basketAccessChallenge)) {
  const user = insecurity.authenticatedUsers.from(req)
  - if (user && id && id !== 'undefined' && user.bid != id) { // eslint-disable-line eqeqeq
  + if (user && id && id !== 'null' && user.bid != id) { // eslint-disable-line eqeqeq
  utils.solve(challenges.basketAccessChallenge)
  }
  }
  