Static relationship collections need to be initialised on subclasses

Will probably make this whole “custom relationships” thing a trait
instead of being on the base class

src/Api/Serializers/DiscussionBasicSerializer.php

@@ -2,6 +2,8 @@
 
 class DiscussionBasicSerializer extends BaseSerializer
 {
+    protected static $relationships = [];
+
     /**
      * The resource type.
      *

src/Api/Serializers/PostBasicSerializer.php

@@ -2,6 +2,8 @@
 
 class PostBasicSerializer extends BaseSerializer
 {
+    protected static $relationships = [];
+
     /**
      * The resource type.
      *