Fix for empty structure content

Author: stefanak-michal

src/packstream/v1/Unpacker.php

@@ -143,7 +143,7 @@ private function unpackStruct(int $marker): array|IStructure|null
             return new $class(...$values);
         } else {
             $this->signature = $signature;
-            return $this->u();
+            return mb_strlen($this->message, '8bit') > $this->offset ? $this->u() : [];
         }
     }