[DomCrawler] Fixed the handling of boolean attributes in ChoiceFormField

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a

An option is marked as selected by the presence of the selected attribute, not by the presence of a non-empty selected attribute. The same is true for checked radio buttons or checkboxes.