minor #46144 [Form] Remove useless condition (MaximePinot)

This PR was merged into the 6.1 branch.

Discussion
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[Form] Remove useless condition

| Q             | A
| ------------- | ---
| Branch?       | 6.1
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | -
| License       | MIT
| Doc PR        | -

Same as #46050.  This condition can be removed since the `$type` parameter is type-hinted: `string $type = null`.

Commits
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064ee4c1a9 Remove useless condition

Author: nicolas-grekas

Form.php

@@ -818,10 +818,6 @@ public function add(FormInterface|string $child, string $type = null, array $opt
 
             $child = (string) $child;
 
-            if (null !== $type && !\is_string($type)) {
-                throw new UnexpectedTypeException($type, 'string or null');
-            }
-
             // Never initialize child forms automatically
             $options['auto_initialize'] = false;