fix: json api instantiation fails

Currently the JsonApi is heavily used with methods `forEndpoint` and
`forResource`, these method set their respective properties, which have
no value initially. When instantiating the class, an "cannot access
before initialization" error because there's no default.

```
Typed property Flarum\Api\JsonApi::$endpointName must not be accessed before initialization
```

framework/core/src/Api/JsonApi.php

@@ -25,8 +25,8 @@ use Tobyz\JsonApiServer\Resource\Resource;
 
 class JsonApi extends BaseJsonApi
 {
-    protected string $resourceClass;
-    protected string $endpointName;
+    protected ?string $resourceClass = null;
+    protected ?string $endpointName = null;
     protected ?Request $baseRequest = null;
     protected ?Container $container = null;