load(documentId) {
    return super.load(documentId).catch((error) => {
      if (this.collection === "users" && error instanceof NotFoundError) {
        return Promise.resolve(null);
      }

      throw error;
    });
  }
If we load a user that does not exists, we have to resolve the promise with null
instead of throwing NotFoundError

@param {string} documentId
@returns {Promise}