fix: 🐛 fallback the schema for mixed properties to string|null instead of null

Author: ttskch

src/JsonSchema/Metadata/Property/Factory/SchemaPropertyMetadataFactory.php

@@ -280,6 +280,7 @@ private function getJsonSchemaFromType(Type $type, ?bool $readableLink = null):
                 TypeIdentifier::OBJECT => ['type' => 'object'],
                 TypeIdentifier::RESOURCE => ['type' => 'string'],
                 TypeIdentifier::CALLABLE => ['type' => 'string'],
+                TypeIdentifier::MIXED => ['type' => 'string'],
                 default => ['type' => 'null'],
             };