This PR was merged into the 5.4 branch.

Discussion
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[Form] Fix return types in form builder

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| 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        |

Fix return types incompatibilities found by psalm in #42511

> Error: The declared return type 'Symfony\Component\EventDispatcher\EventDispatcherInterface' for Symfony\Component\Form\ButtonBuilder::getEventDispatcher is not nullable, but 'null' contains null (see https://psalm.dev/144)
Error: The declared return type 'string' for Symfony\Component\Form\ButtonBuilder::getAction is not nullable, but 'null' contains null (see https://psalm.dev/144)
Error: The declared return type 'string' for Symfony\Component\Form\ButtonBuilder::getMethod is not nullable, but 'null' contains null (see https://psalm.dev/144)
Error: The declared return type 'Symfony\Component\Form\RequestHandlerInterface' for Symfony\Component\Form\ButtonBuilder::getRequestHandler is not nullable, but 'null' contains null (see https://psalm.dev/144)
Error: The declared return type 'Symfony\Component\Form\FormFactoryInterface' for Symfony\Component\Form\FormConfigBuilder::getFormFactory is not nullable, but 'Symfony\Component\Form\FormFactoryInterface|null' contains null (see https://psalm.dev/144)
Error: The declared return type 'string' for Symfony\Component\Form\FormConfigBuilder::getAction is not nullable, but 'null|string' contains null (see https://psalm.dev/144)

Commits
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ea606ea [Form] Fix return types in form builder