I have not used Infinite Campus, but looking at the guide for syncing to Infinite Campus, it does appear to require at least one grade is entered in order for the sync to work: https://community.canvaslms.com/t5/Infinite-Campus-and-Canvas/How-do-I-sync-my-course-gradebook-from...
In my searching, I have not found a workaround for this issue. However, I did find some suggestions, based on the idea that the Due Date has passed, but you have not entered grades yet. You can set up your course to automatically assign a grade for missing work (typically 0%) in the Gradebook Settings. If you have nightly sync turned on, the first time that sync occurs after the due date, anyone who has not turned it in will receive a 0%, which is sent to IC. I suspect (if like PowerSchool), then students who have turned it in will see a blank entry in IC. This forces a grades for missing work, which therefore forces the sync to occur.
Maybe someone more familiar with IC might be aware of another option. This is what I can find.