Found solution (workaround):
AS mentioned above, the PPF:ResourceDisplay custom field is updated when a resource is added/removed from a Work Item. However, this does nothing otherwise and shows no data if the Resource is blank. However, when the Resource of a task is blank, ownership of the task and its completion falls on the OWNER of the task, typically the PM of the project.
The solution I found was to create a workflow rule that will update the PPR: ResourceDisplay field, not only when a resource is updated but if the Period Project Report custom action is triggered. I set a custom toggle to be triggered to TRUE, which triggers the workflow. The workflow runs on the Work Item object and when triggered, Updates the PPR: ResourceDisplay field with all of the Resources linked to the work item. IF the Resource link is NULL/Blank, then the user in the Owner field is stored in the PPR: ResourceDisplay text field and this field is exposed in the Document Publisher template.