IcingaApiCommandRenderer: Don't set empty array, if no attr is set

- Use the returned value instead, either array or null.

- With https://github.com/Icinga/icinga2/issues/8167 attrs=[] means that no attrs are
  fetched at all, and attrs=null means that all available attrs are fetched

library/Icingadb/Command/Renderer/IcingaApiCommandRenderer.php

@@ -127,7 +127,7 @@ class IcingaApiCommandRenderer implements IcingaCommandRendererInterface
     {
         $data = [
             'all_joins' => 1,
-            'attrs'     => $command->getAttributes() ?: []
+            'attrs'     => $command->getAttributes()
         ];
 
         $endpoint = 'objects/' . $this->getObjectPluralType($this->applyFilter($data, $command->getObjects()));