When activating the incident rules I am presented with the option to move a number of incidents in state with ID value of Null to another rule state.
Oracle Service Cloud
Records in a Null state will never be affected by rules. You must move the records to another state other than Null. These apply to any type of records that a business rule can be created for. Incidents, answers, contacts, organization rules are a few examples.
When you move these records to another state the rules do not run on those records until the records are updated. Basically it just moves the records into a rule state so if they are ever edited again the rules will be able to execute on the records If a record remains in a Null rule state then the rules will not ever run on the incident as long as it remains in that Null state.
The only thing that will move the records out of the Null state is a direct database update or if you move them when prompted at the time of activating the rules. In the case of incidents, it is best not to move the incidents to an initial state since those rules are generally used for newly created incidents and you do not want the incidents to go through the initial rules the next time you edit any of the incidents. We recommend moving these incidents to one of your in progress or solved rule states. Then if those incidents are ever edited again the rules will have a chance to run on the incidents.