Edit Output Style > Bibliography > Field Substitions now shows one of four options:
If the Volume and Pages fields are empty, use:<optional field>.
Add new option:
If the Pages field is empty, use:<optional field>.
or split up current option into two options:
If the Volume field is empty, use:<optional field>.
If the Pages field is empty, use:<optional field>.
Why: Some Journal Style definitions require, if pages field is empty, use Article number (Custom 7 in EndNote).