bug #39603 [TwigBridge] allow null values in form helpers (xabbuh)

This PR was merged into the 5.2 branch.

Discussion
----------

[TwigBridge] allow null values in form helpers

| Q             | A
| ------------- | ---
| Branch?       | 5.2
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #39591| License       | MIT
| Doc PR        |

Commits
-------

f908047c6b allow null values in form helpers

Author: nicolas-grekas

Extension/FormExtension.php

@@ -111,7 +111,7 @@ public function getFieldValue(FormView $view)
         return $view->vars['value'];
     }
 
-    public function getFieldLabel(FormView $view): string
+    public function getFieldLabel(FormView $view): ?string
     {
         return $this->createFieldTranslation(
             $view->vars['label'],
@@ -120,7 +120,7 @@ public function getFieldLabel(FormView $view): string
         );
     }
 
-    public function getFieldHelp(FormView $view): string
+    public function getFieldHelp(FormView $view): ?string
     {
         return $this->createFieldTranslation(
             $view->vars['help'],
@@ -164,7 +164,7 @@ private function createFieldChoicesList(iterable $choices, $translationDomain):
         }
     }
 
-    private function createFieldTranslation(?string $value, array $parameters, $domain): string
+    private function createFieldTranslation(?string $value, array $parameters, $domain): ?string
     {
         if (!$this->translator || !$value || false === $domain) {
             return $value;