Is there a way to show null values in a Summary Report

There are two ways you could accomplish this.

1. Create a new formula numeric field which basically convert the existing value into a numeric value even for now by doing a NZ. Use that new formula numeric field in your summary report and that should display.

2. The other way to try is to Modify the field properties of the summary field to treat blank values as zero. Make sure that you are not filtering out any records in your summary table based on the field value being zero.