fix(openapi): invalid hydra type specification

Author: DjordyKoert

features/openapi/docs.feature

@@ -238,7 +238,8 @@ Feature: Documentation support
                                         "type": "string"
                                     },
                                     "property": {
-                                        "type": ["string", "null"]
+                                        "type": "string",
+                                        "nullable": true
                                     },
                                     "required": {
                                         "type": "boolean"

src/Hydra/JsonSchema/SchemaFactory.php

@@ -92,12 +92,12 @@ public function buildSchema(string $className, string $format = 'jsonld', string
             unset($schema['items']);
 
             switch ($schema->getVersion()) {
-                // JSON Schema + OpenAPI 3.1
                 case Schema::VERSION_OPENAPI:
+                    $nullableStringDefinition = ['type' => 'string', 'nullable' => true];
+                    break;
                 case Schema::VERSION_JSON_SCHEMA:
                     $nullableStringDefinition = ['type' => ['string', 'null']];
                     break;
-                    // Swagger
                 default:
                     $nullableStringDefinition = ['type' => 'string'];
                     break;