Allow relationship without requirement of 'data'

src/Document.php

@@ -80,6 +80,10 @@ protected function getIncluded(ElementInterface $element, $includeParent = false
             foreach ($resource->getUnfilteredRelationships() as $relationship) {
                 $includedElement = $relationship->getData();
 
+                if (! $includedElement instanceof ElementInterface) {
+                    continue;
+                }
+
                 foreach ($this->getIncluded($includedElement, true) as $child) {
                     // If this resource is the same as the top-level "data"
                     // resource, then we don't want it to show up again in the

src/Relationship.php

@@ -19,24 +19,24 @@ class Relationship
     /**
      * The data object.
      *
-     * @var ElementInterface
+     * @var ElementInterface|null
      */
     protected $data;
 
     /**
      * Create a new relationship.
      *
-     * @param ElementInterface $data
+     * @param ElementInterface|null $data
      */
-    public function __construct(ElementInterface $data)
+    public function __construct(ElementInterface $data = null)
     {
         $this->data = $data;
     }
 
     /**
      * Get the data object.
      *
-     * @return ElementInterface
+     * @return ElementInterface|null
      */
     public function getData()
     {
@@ -46,7 +46,7 @@ public function getData()
     /**
      * Set the data object.
      *
-     * @param ElementInterface $data
+     * @param ElementInterface|null $data
      * @return $this
      */
     public function setData($data)

src/Resource.php

@@ -271,7 +271,10 @@ protected function buildRelationships()
             $relationship = $this->serializer->getRelationship($this->data, $name);
 
             if ($relationship) {
-                $relationship->getData()->with($nested)->fields($this->fields);
+                $relationshipData = $relationship->getData();
+                if ($relationshipData instanceof ElementInterface) {
+                    $relationshipData->with($nested)->fields($this->fields);
+                }
 
                 $relationships[$name] = $relationship;
             }