Set creator to not nullable

As a follow on to #5154, the following tables should also have creator as not nullable.

- application_exceptions 260
- applications 0
- designation_agreements 0
- education_programs 128
- institution_users 21
- sequence_controls 1436
- student_appeal_requests 0
- student_files 80
- student_loan_balances 20
- students 238
- student_scholastic_standings 0
- supporting_users 205

Locate the schema or migration entry points for the twelve listed tables and compare their current creator definitions with the change in #5154. Review the existing creator-null counts first, then determine the project’s established approach for making each creator field non-null. Done means all listed tables enforce the requested constraint and the relevant migration checks pass.

Written by the indexing model from the issue text.