minor symfony#61675 pass the empty string instead of null as key to array_key_exists() (xabbuh)

This PR was merged into the 6.4 branch.

Discussion
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 pass the empty string instead of null as key to array_key_exists()

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

Commits
-------

8efcf72 pass the empty string instead of null as key to array_key_exists()

Author: fabpot

src/Symfony/Component/Form/ChoiceList/ArrayChoiceList.php

@@ -114,7 +114,7 @@ public function getChoicesForValues(array $values): array
         $choices = [];
 
         foreach ($values as $i => $givenValue) {
-            if (\array_key_exists($givenValue, $this->choices)) {
+            if (\array_key_exists($givenValue ?? '', $this->choices)) {
                 $choices[$i] = $this->choices[$givenValue];
             }
         }