add nullable type support

?string - type: [string, null] not type: string

Author: Сергей Лавриеня

src/Generator.php

@@ -137,14 +137,14 @@ protected function generateProperty(PropertyInterface $property): ?Property
 
         $required = $default === null && !$type->allowsNull();
         if ($type->isBuiltin()) {
-            return new Property($type->getName(), $options, $title, $description, $required, $default, $format);
+            return new Property($type->getName(), $options, $title, $description, $required, $type->allowsNull(), $default, $format);
         }
 
         // Class or enum
         $class = $type->getName();
 
         return \is_string($class) && \class_exists($class)
-            ? new Property($class, [], $title, $description, $required, $default, $format)
+            ? new Property($class, [], $title, $description, $required, $type->allowsNull(), $default, $format)
             : null;
     }
 }

src/Schema/Property.php

@@ -20,6 +20,7 @@ public function __construct(
         public readonly string $title = '',
         public readonly string $description = '',
         public readonly bool $required = false,
+        public readonly bool $allowsNull = false,
         public readonly mixed $default = null,
         public readonly ?Format $format = null,
     ) {
@@ -61,7 +62,7 @@ public function jsonSerialize(): array
             return $property;
         }
 
-        $property['type'] = $this->type->value;
+        $property['type'] = $this->allowsNull ? [$this->type->value, Type::Null] : $this->type->value;
 
         if ($this->type === Type::Array) {
             if (\count($this->options) === 1) {