minor #36128 [Form][CheckboxType] Remove _false_is_empty flag (fancyweb)

This PR was merged into the 5.1-dev branch.

Discussion
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[Form][CheckboxType] Remove _false_is_empty flag

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Cleanup of https://github.com/symfony/symfony/pull/35938.

Commits
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6fac6d4086 [Form][CheckboxType] Remove _false_is_empty flag

src/Symfony/Component/Form/Extension/Core/Type/CheckboxType.php

@@ -33,7 +33,6 @@ class CheckboxType extends AbstractType
         // doing so also calls setDataLocked(true).
         $builder->setData(isset($options['data']) ? $options['data'] : false);
         $builder->addViewTransformer(new BooleanToStringTransformer($options['value'], $options['false_values']));
-        $builder->setAttribute('_false_is_empty', true); // @internal - A boolean flag to treat false as empty, see Form::isEmpty() - Do not rely on it, it will be removed in Symfony 5.1.
     }
 
     /**

src/Symfony/Component/Form/Form.php

@@ -740,9 +740,7 @@ class Form implements \IteratorAggregate, FormInterface, ClearableErrorsInterfac
             // arrays, countables
             ((\is_array($this->modelData) || $this->modelData instanceof \Countable) && 0 === \count($this->modelData)) ||
             // traversables that are not countable
-            ($this->modelData instanceof \Traversable && 0 === iterator_count($this->modelData)) ||
-            // @internal - Do not rely on it, it will be removed in Symfony 5.1.
-            (false === $this->modelData && $this->config->getAttribute('_false_is_empty'));
+            ($this->modelData instanceof \Traversable && 0 === iterator_count($this->modelData));
     }
 
     /**