Revert "Don't include empty schema"

This reverts commit 8235126.

Author: tobyzerner

src/OpenApi/OpenApiGenerator.php

@@ -95,12 +95,8 @@ private function buildSchema(Resource $resource, array $schema, array $overrides
                 'properties' => [
                     'type' => ['type' => 'string', 'const' => $resource->type()],
                     'id' => ['type' => 'string'],
-                    'attributes' => !empty($schema['attributes'])
-                        ? ['type' => 'object'] + $schema['attributes']
-                        : [],
-                    'relationships' => !empty($schema['relationships'])
-                        ? ['type' => 'object'] + $schema['relationships']
-                        : [],
+                    'attributes' => ['type' => 'object'] + ($schema['attributes'] ?? []),
+                    'relationships' => ['type' => 'object'] + ($schema['relationships'] ?? []),
                 ],
             ],
             $overrides,