Fix wrong DateTransformer timezone param for non-UTC configuration

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | N/A
| Fixed tickets | #12808 
| License       | MIT
| Doc PR        | none

This PR is a fix of a little mistake on PR #12404.

Pass 'UTC' param cause issue on single_text date type, `null` should be passed to get the default php timezone.

Don't know how to add test for #12808 issue, do someone know how to "change" properly timezone during tests? Where can I do this? 

Thanks.