17244 WP attributes with zero value are not shown in split view

[``* `#17244` Work package attributes with zero value are not shown in split view``](https://community.openproject.org/work_packages/17244)
@hschink typo here in `propery`.

Also `property == null` would suffice if you do decide to use the `==` operator (we can tell JSHint/@houndci not to warn about this, if you prefer).