IBX-2264: Handled empty Thumbnail ProxyObject (#91)

Author: barw4

src/lib/Server/Output/ValueObjectVisitor/Version.php

@@ -136,7 +136,7 @@ private function visitThumbnail(
     ): void {
         $generator->startObjectElement('Thumbnail');
 
-        if (!empty($thumbnail)) {
+        if (!empty($thumbnail->resource)) {
             $generator->startValueElement('resource', $thumbnail->resource);
             $generator->endValueElement('resource');