Fixed the handling of boolean attributes in ChoiceFormField

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.
bug #10925 [DomCrawler] Fixed the handling of boolean attributes in ChoiceFormField (stof)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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b505708 Fixed the handling of boolean attributes in ChoiceFormField