When a scheduled reported is emailed as an attachment (ex. .pdf file), the attachment contains no data. Here is a screen shot:
The report (sys_report record) did not have the necessary role when the schedule report was executed.
Update the 'Roles field in the report with the necessary role. This is the role required to view the report.
Reports > Administration > (open report) > update 'Roles' field with necessary role (ex. 'itil').