code: function existsEvaluator(condition, userAttributes) {
    var userValue = userAttributes[condition.name];
    return typeof userValue !== 'undefined' && userValue !== null;
  }
docs: Evaluate the given exists match condition for the given user attributes
  @param   {Object}  condition
  @param   {Object}  userAttributes
  @returns {Boolean} true if both:
  1) the user attributes have a value for the given condition, and
  2) the user attribute value is neither null nor undefined
  Returns false otherwise