[10.x] Fix parentOfParameter method (#49548)

Author: iamgergo

src/Illuminate/Routing/Route.php

@@ -583,7 +583,7 @@ public function parentOfParameter($parameter)
     {
         $key = array_search($parameter, array_keys($this->parameters));
 
-        if ($key === 0) {
+        if ($key === 0 || $key === false) {
             return;
         }