Use {"nullable": true, "anyOf": [{"$ref": ...}]} to comply with OpenAPI 3.0

OpenAPI 3.1 is not yet released, but fixes nullability in
the way we had fixed it before (via `{"oneOf": [{"type": "null"}, ...]}`) in
OAI/OpenAPI-Specification#1977.

Until OpenAPI 3.1 is released, things like ``{"type": ["integer", "null"]}` are
not valid definitions (because `"null"` is not yet a recognized type).

We'll stick to OpenAPI 3.0 for now, using:

 * `{"nullable": true, ...}` for simple types
 * `{"nullable": true, "anyOf": [{"$ref": ...}]}` for type references

Author: Ocramius

TypeFactory.php

@@ -180,11 +180,13 @@ private function addNullabilityToTypeDefinition(array $jsonSchema, Type $type, a
             return $jsonSchema;
         }
 
-        return [
-            'oneOf' => [
-                ['type' => 'null'],
-                $jsonSchema,
-            ],
-        ];
+        if (\array_key_exists('$ref', $jsonSchema)) {
+            return [
+                'nullable' => true,
+                'anyOf' => [$jsonSchema],
+            ];
+        }
+
+        return array_merge($jsonSchema, ['nullable' => true]);
     }
 }