bug #873 [AiBundle] Fix fatal error when a value is not defined (lyrixx)

This PR was merged into the main branch.

Discussion
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[AiBundle] Fix fatal error when a value is not defined

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Docs?         | no
| Issues        |
| License       | MIT

### Before

<img width="1285" height="805" alt="image" src="https://github.com/user-attachments/assets/57000bd8-1dd1-4939-9f8a-f390dc9b64ac" />

### After

<img width="1285" height="805" alt="image" src="https://github.com/user-attachments/assets/1a8a97fb-3dd6-4259-b60b-362ea8e5d5bf" />

Commits
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f14eaeb [AiBundle] Fix fatal error when a value is not defined

Author: chr-hertel

src/ai-bundle/config/options.php

@@ -882,7 +882,7 @@
                             ->end()
                             ->beforeNormalization()
                                 ->always(function ($v) {
-                                    if (\is_string($v)) {
+                                    if (\is_string($v) || null === $v) {
                                         return $v;
                                     }