If there are no available shipment promotion offer ids, the site crashes

Problem/Motivation

in PromotionSubscriber::getPromotions we first check what offer_ids are applicable, and filter out only the ones that implement ShipmentPromotionOfferInterface.

If this is exactly 0, the function getOfferIds will return an empty array.

It is the passed to loadAvailable from PromotionStorage. If the array is empty, then this check will not apply:

if ($offer_ids) {
  $query->condition('offer.target_plugin_id', $offer_ids, 'IN');
}

Leading us to load all available promotions (not only the ones that implements ShipmentPromotionOfferInterface). Later the site will then crash either because of this (depending on the php.ini values for assertions):

assert($offer instanceof ShipmentPromotionOfferInterface);

Or because the offer plugin does not have a method called isDisplayInclusive

Error: Call to undefined method MyOfferPlugin::isDisplayInclusive()

Either way, the site crashes, and it might not be possible to check out.

Steps to reproduce

- Make sure you have no shipping promotion plugins
- Make sure you have some non-shipping promotion plugins
- Make sure you have some promotion created
- Go to the checkout

Proposed resolution

Make sure we do not try to load non-relevant promotions

Remaining tasks

Write tests.
Review. Commit.

User interface changes

None.

API changes

None.

Data model changes

None.