While it is possible to set the assignment group in an inbound field rule, I wouldn’t recommend it unless you want the incident getting automatically resolved or sent to a different escalation policy in PagerDuty. That’s because there are business rules that perform such operations automatically when there is a change in the assignment group.
To elaborate: if the other assignment group is provisioned to PagerDuty (meaning, it has an escalation policy associated with it), then the integration will delegate the incident to the other escalation policy, returning it to the triggered state and paging the people in that escalation policy. Otherwise (if the group is not provisioned), the it will resolve the PagerDuty incident and mark it as “unlinked” in ServiceNow, as it is at that point considered to be handed off to group that is working outside of PagerDuty.
If this is the intended effect, then (and correct me my assumptions are incorrect) there is less need to blank out the assignment field, because when the next person in PagerDuty acknowledges the incident (assuming that you have designed the lookup script in the inbound field rule to exclude certain cases in acknowledgment), then the incident will get automatically assigned to them in ServiceNow.