✅(backend) add test coverage for blank sub behavior

Update the test to document the actual contract without modifying the
underlying model behavior.

These tests act as non-regression coverage and explicitly assert that
users may have a null sub or email. They are intended to document the
current behavior of the initial user model rather than evolve or
constrain it.

Ghost rows have not been reported as an operational issue. For the sake
of simplicity, avoid changing the model unless required by a concrete
issue or unless the benefits clearly outweigh the added complexity.

src/backend/core/tests/test_models_users.py

@@ -94,3 +94,23 @@ def test_models_users_sub_null_email_null_does_not_prevent_creation():
     u1 = factories.UserFactory(sub=None, email=None)
     u2 = factories.UserFactory(sub=None, email=None)
     assert u1.pk != u2.pk
+
+
+def test_models_users_sub_can_be_null():
+    """sub is nullable: pending users exist before OIDC activation."""
+    user = factories.UserFactory(sub=None)
+    user.refresh_from_db()
+    assert user.sub is None
+
+
+def test_models_users_sub_null_does_not_prevent_creation():
+    """Multiple users can be created with sub=None (pending state)."""
+    u1 = factories.UserFactory(sub=None)
+    u2 = factories.UserFactory(sub=None)
+    assert u1.pk != u2.pk
+
+
+def test_models_users_sub_blank_is_accepted():
+    """sub='' passes validation because blank=True; null is preferred but not enforced."""
+    user = factories.UserFactory.build(sub="")
+    user.full_clean()