Issue #2995222 by e0ipso: Tolerate null as output for SingleNestedEnhancer

src/Plugin/jsonapi/FieldEnhancer/SingleNestedEnhancer.php

@@ -62,7 +62,10 @@ class SingleNestedEnhancer extends ResourceFieldEnhancerBase {
    */
   public function getOutputJsonSchema() {
     return [
-      'type' => 'string',
+      'oneOf' => [
+        ['type' => 'string'],
+        ['type' => 'null'],
+      ],
     ];
   }