bug #261 [Twig][Live] null attribute values render without value (ie autofocus) (kbond)

This PR was merged into the 2.x branch.

Discussion
----------

[Twig][Live] null attribute values render without value (ie autofocus)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tickets       | n/a
| License       | MIT

Ref: symfony/ux#220 (comment)

A null attribute value renders without the _value_:

```twig
{# templates/components/my_component.html.twig #}
<input{{ attributes}}/>

{# render component #}
{{ component('my_component', { type: 'text', value: '', autofocus: null }) }}

{# renders as: #}
<input type="text" value="" autofocus/>
```

**TODO:**
- [x] `null` vs `''` (symfony/ux#220 (comment))
- [x] attributes without a value like `autofocus` (symfony/ux#220 (comment))
- [x] LiveComponent dehydration/hydration issues (`null` values are lost during re-render) - solved in #264

Commits
-------

c1e484b [Twig][Live] null attribute values render without value (ie autofocus)

Author: weaverryan

tests/Integration/LiveComponentHydratorTest.php

@@ -274,7 +274,7 @@ public function testCanDehydrateAndHydrateComponentsWithAttributes(): void
         $factory = self::getContainer()->get('ux.twig_component.component_factory');
 
         /** @var ComponentWithAttributes $component */
-        $component = $factory->create('with_attributes', $attributes = ['class' => 'foo']);
+        $component = $factory->create('with_attributes', $attributes = ['class' => 'foo', 'value' => null]);
 
         $this->assertSame($attributes, $component->attributes->all());