Correcting validation of form field to set email notification address upon user registration

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/zikula/core/issues/1840
| Refs tickets  | -
| License       | MIT
| Doc PR        | -