[Form] Added radio button for empty value to expanded single-choice fields
merged branch bschussek/issue3154 (PR #7939)

This PR was merged into the master branch.

Discussion
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[Form] Added radio button for empty value to expanded single-choice fields

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3154
| License       | MIT
| Doc PR        | symfony/symfony-docs#2605

Commits
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7933971 [Form] Added radio button for empty value to expanded single-choice fields
fixed CS