[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
@Taluu not exactly the same. ``isset($array[$key])`` covers more cases that ``$array[$key] !== null``. It covers ``null`` values, but not missing values. So ``$array[$key] !== null`` is indeed not necessary here, but both conditions are not the same thing.

@Nek- please also switch the order of operands for each condition. We are using Yoda conditions in Symfony