Add nullable scratch test

tests/Fixtures/Scratch/Nullable.php

@@ -0,0 +1,58 @@
+<?php declare(strict_types=1);
+
+/**
+ * @license Apache 2.0
+ */
+
+namespace OpenApi\Tests\Fixtures\Scratch;
+
+use OpenApi\Attributes as OAT;
+
+#[OAT\Info(
+    title: 'Nullable',
+    version: '1.0'
+)]
+class Api
+{
+}
+
+#[OAT\Schema(
+    type: 'string',
+    format: 'rfc3339-timestamp',
+    externalDocs: new OAT\ExternalDocumentation(
+        description: '**RFC3339** IETF',
+        url: 'https://tools.ietf.org/html/rfc3339'
+    ),
+    example: '2023-08-02T07:06:46+03:30'
+)]
+class MyDateTime
+{
+}
+
+#[OAT\Schema]
+class Nullable
+{
+    #[OAT\Property]
+    public ?string $firstname;
+
+    #[OAT\Property(nullable: true)]
+    public ?string $lastname;
+
+    #[OAT\Property]
+    public ?MyDateTime $birthdate;
+
+    #[OAT\Property(nullable: true)]
+    public MyDateTime $otherdate;
+
+    #[OAT\Property]
+    public MyDateTime|null $anotherdate;
+}
+
+#[OAT\Get(
+    path: '/api/endpoint',
+    description: 'An endpoint',
+    responses: [new OAT\Response(response: 200, description: 'OK')]
+)]
+class NullableEndpoint
+{
+}

tests/Fixtures/Scratch/Nullable.yaml

@@ -0,0 +1,44 @@
+openapi: 3.0.0
+info:
+  title: Nullable
+  version: '1.0'
+paths:
+  /api/endpoint:
+    get:
+      description: 'An endpoint'
+      responses:
+        '200':
+          description: OK
+components:
+  schemas:
+    MyDateTime:
+      type: string
+      format: rfc3339-timestamp
+      externalDocs:
+        description: '**RFC3339** IETF'
+        url: 'https://tools.ietf.org/html/rfc3339'
+      example: '2023-08-02T07:06:46+03:30'
+    Nullable:
+      properties:
+        firstname:
+          type: string
+          nullable: true
+        lastname:
+          type: string
+          nullable: true
+        birthdate:
+          nullable: true
+          oneOf:
+            -
+              $ref: '#/components/schemas/MyDateTime'
+        otherdate:
+          nullable: true
+          oneOf:
+            -
+              $ref: '#/components/schemas/MyDateTime'
+        anotherdate:
+          nullable: true
+          oneOf:
+            -
+              $ref: '#/components/schemas/MyDateTime'
+      type: object