How to display a record in summary report even if there is no item in it?

Summary reports are not setup to show rows with no records being summarized. The only way that you would be able to do this is to have Risk Impact as its' own table. On that table, you would then do one summary field for each Risk Probability, using the filter in the Summary to limit the records to that Risk Probability. Once you have that done, you can do a Table Report with the Risk Impact as the Row and the summary fields as the columns.