code: function value(utils) {
    const target = this._obj,
          value = target.value || null;
    isValidation(target);
    this.assert(null !== value,
        '#{this} should have value',
        '#{this} should not have value'
    );
    utils.flag(this, 'object', value);
  }
docs: Assert that target contains a value.
  Mutates current chainable object to be target.value.
  @example
  expect(target).to.[not].have.a.value
  target.should.[not].have.a.value