Incorrect return type in loadEnabledByCode definition.

Describe your bug or feature request.

The CouponStorageInterface defines the return type of the method loadEnabledByCode as CouponInterface, but the method returns FALSE if no such code is found.

I would return NULL, if no coupon is found (like EntityStorageInterface::load()) and change the return type to
* @return \Drupal\commerce_promotion\Entity\CouponInterface|null.