[Form] Fixed #11675 ValueToDuplicatesTransformer accept "0" value

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #11675
| License       | MIT
@stof Yup, that's what I meant (the null comparision is done by the `isset` verification). My bad if there was a misunderstanding. :)