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)