create_preregistration_user: Add additional hardening assertion.

TestMaybeSendToRegistration needs tweaking here, because it wasn't
setting the subdomain for the dummy request, so
maybe_send_to_registration was actually running with realm=None, which
is not right for these tests.

Also, test_sso_only_when_preregistration_user_exists was creating
PreregistrationUser without setting the realm, which was also incorrect.

zerver/views/auth.py

@@ -121,6 +121,7 @@ def create_preregistration_user(
     full_name_validated: bool = False,
 ) -> PreregistrationUser:
     assert not (realm_creation and realm is not None)
+    assert not (realm is None and not realm_creation)
 
     return PreregistrationUser.objects.create(
         email=email,