if you do not have any resources then you can not compare and % completes how do you intend to measure you have nothing to measure against. The only thing that you can measure is % complete as it relates to time and it has to be on a linear basis. For example you have a project it is 100 days longs so if you are on schedule on day 50 you should be 50% complete that would be your planned % complete. But lets say that on day 50 due to the lack of progress that now your project is projected to complete in 120 days so you can say that you are 20 days behind schedule so that means that day 50-20 days behind schedule you only earned 30 days. So now you planned % complete was 50% but now you are only 30% complete.
This is in its simplistic terms on a straight line approach and not having a measuring stick such as manhours and or dollars as tool to measure by makes it difficult to measure anything except time.
If looking at the summary Schedule % complete it uses (BCWS*100)/BAC which is the Budgeted Cost for Work Scheduled times 100 divided by the Budgeted At Completion Costs.
The Summary Performance % complete uses (BCWP*100)/BAC which is the Budgeted Cost for Work Performed times 100 divided by the Budgeted At Completion Costs.
since you have no resource you will get zero
There is a tool called project tracker that will give you the percentages without resources but is based on a linear progression of time as I have stated in the first part of this post.
here is the link
http://www.willmer.co.uk/projecttracker