You must create a migration, where you will specify default value for a new field, since you don't want it to be null. If null is not required, simply add null=True and create and run migration.
Go to the migrations folder and delete manually files that have 000*_lastAction_blah-blah type of name, you can delete, probably all, but 0001_initial.py file. After that run ./manage.py make migrations app_you_are_updateing, it should update your database.