If no parent is set and I call a siblings collection, return a new collection

Fix for a plugins combination issue detailed in
https://github.com/getgrav/grav-plugin-sitemap/issues/22

system/src/Grav/Common/Page/Page.php

@@ -2337,6 +2337,9 @@ class Page
                             $results = $collection->addPage($this->parent());
                             break;
                         case 'siblings':
+                            if (!$this->parent()) {
+                                return new Collection();
+                            }
                             $results = $this->parent()->children()->remove($this->path());
                             break;
                         case 'descendants':