Refuse a pre-registration that selects nothing

[vikrantwiz02]

What happens today

submit_preregistration marks the student complete regardless of what was submitted:

reg_check, created = StudentRegistrationChecks.objects.get_or_create(
    student_id=student, semester_id_id=next_semester.id,
    defaults={'pre_registration_flag': True})

Nothing checks that registrations or backlog_registrations contained anything. A student who submits the form without choosing courses therefore ends up with zero saved choices and the flag set — and because the page returns an existing registration before it offers the form again, they are locked out of a registration they never made. The only way back is a database edit; it has already caught students in the current intake.

Change

An empty submission is refused:

400  Select at least one course before submitting your pre-registration.

No rows, no flag, so the student simply picks their courses and submits.

Verified against a first-semester student, rolled back afterwards:

submission	response	rows / flags after
no courses	400 with the message above	(0, 0) — nothing written
one course	201 {"status": "success"}	(1, 1)

Note for whoever is running the current window

This prevents new cases; it does not release students already stuck. Those have a StudentRegistrationChecks row with the flag set and no matching InitialRegistration rows, and deleting that row lets them register again. Scope any such cleanup to the current batch — the same pattern matches thousands of older rows whose choices were legitimately consumed by allotment.

manage.py check clean, no migrations.