Fix related and relationships endpoints for disabled resources

1. +++ b/src/Controller/EntityResource.php
   @@ -168,6 +179,9 @@ class EntityResource {
   +   * @throws \Drupal\Core\Entity\EntityStorageException
   
   @@ -214,6 +228,9 @@ class EntityResource {
   +   * @throws \Drupal\Core\Entity\EntityStorageException
   
   @@ -251,6 +268,9 @@ class EntityResource {
   +   * @throws \Drupal\Core\Entity\EntityStorageException
   

   Maybe we should catch and wrap this exception? That would allow us to provide more context about the error in our response.

2. +++ b/src/Controller/EntityResource.php
   @@ -346,8 +369,20 @@ class EntityResource {
   +    //though the normalizer skips disabled references, we can avoid unnecessary
   +    //work by checking here too.
   

   Nit: missing spaces

   Perhaps we should find a way to combine these checks?

3. +++ b/src/Controller/EntityResource.php
   @@ -382,13 +417,53 @@ class EntityResource {
   -    if (!($field_list = $entity->get($related_field)) || !$this->isRelationshipField($field_list)) {
   ...
   +    $field_list = $entity->get($related_field);
   +    $this->validateReferencedResource($field_list, $related_field);
   ...
   +  protected function validateReferencedResource(EntityReferenceFieldItemListInterface $field_list, $related_field) {
   +    if (!$field_list || !$this->isRelationshipField($field_list)) {
   

   I think this will throw an exception if $field_list is NULL|FALSE (even though we're checking for it in validateReferencedResource) because you've type-hinted to the interface.

4. +++ b/src/Normalizer/RelationshipNormalizer.php
   @@ -72,6 +72,11 @@ class RelationshipNormalizer extends NormalizerBase {
   +      // If the relationship points to a disabled resource type, do not add the
   +      // normalized relationship item.
   +      if (!$relationship_item->getTargetResourceType()) {
   +        continue;
   +      }
   

   nice

Patch looks good to me. Let's create that follow up or add it to the scope of #2934362: Specify a "code" for every exception that JSON API throws.

+++ b/src/Controller/EntityResource.php
@@ -434,7 +434,7 @@ class EntityResource {
-  protected function validateReferencedResource(EntityReferenceFieldItemListInterface $field_list, $related_field) {
+  protected function validateReferencedResource($field_list, $related_field) {
     if (!$field_list || !$this->isRelationshipField($field_list)) {

Note that this could use
assert(is_null($field_list) || $field_list instanceof EntityReferenceFieldItemListInterface);

Any reason to not do that?

Then we at least keep the strict type checking :)

Yep, that was an infra fail. #13 is green now :)