We ran into the same issue when we tried to create a business rule with a user just clicking OK with no justification.
In CE 4.1, there is a workaround to prevent blank justifications to not fire, which is below.
1) Create the filter as shown in the below image and click on the "Filter Query" button
2) Here is the screenshot of the filter query view
3) Select the keyword "empty"
4) Remove the keyword "empty" as shown below and click on the load button.
5) Finally click on the save button to save the business rule.
In CE 4.2 (currently in beta), there will be a new filter value for 'Is Empty', which will negate the above workaround.