fix #10054 - form data collector with dynamic fields

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10054
| License       | MIT
| Doc PR        | -

src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php

@@ -139,7 +139,9 @@ class FormDataCollector extends DataCollector implements FormDataCollectorInterf
         $hash = spl_object_hash($form);
 
         if (!isset($this->dataByForm[$hash])) {
-            $this->dataByForm[$hash] = array();
+            // field was created by form event
+            $this->collectConfiguration($form);
+            $this->collectDefaultData($form);
         }
 
         $this->dataByForm[$hash] = array_replace(