minor #16697 CS: remove impossible default argument value (keradus)

This PR was merged into the 2.7 branch.

Discussion
----------

CS: remove impossible default argument value

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ?
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Commits
-------

acef3a3 CS: remove impossible default argument value

Author: fabpot

Extension/DataCollector/FormDataCollector.php

@@ -217,7 +217,7 @@ public function getData()
         return $this->data;
     }
 
-    private function recursiveBuildPreliminaryFormTree(FormInterface $form, &$output = null, array &$outputByHash)
+    private function recursiveBuildPreliminaryFormTree(FormInterface $form, &$output, array &$outputByHash)
     {
         $hash = spl_object_hash($form);
 
@@ -236,7 +236,7 @@ private function recursiveBuildPreliminaryFormTree(FormInterface $form, &$output
         }
     }
 
-    private function recursiveBuildFinalFormTree(FormInterface $form = null, FormView $view, &$output = null, array &$outputByHash)
+    private function recursiveBuildFinalFormTree(FormInterface $form = null, FormView $view, &$output, array &$outputByHash)
     {
         $viewHash = spl_object_hash($view);
         $formHash = null;