Fix condition in rest_page_ordering method

[szepeviktor]

Description of the Change

e.g. $previd could never be null

Verification Process

Manually by me.

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests passed.

Changelog Entry

Fixed - Condition in rest_page_ordering

Props @szepeviktor