Don't create GradeSync with no GradeSyncGrades

This is mostly a possibility while testing locally. GradeSync with not
GradeSyncGrades will currently stay "in_progress" forever as no
GradeSync will ever finish.

We could make changes so this type of sync is marked as finished
immediately but we are better off not creating them in the first place.

lms/views/dashboard/api/grading.py

@@ -53,6 +53,12 @@ def create_grading_sync(self):
         sync_lms_users = self.db.scalars(
             select(LMSUser).where(LMSUser.h_userid.in_(sync_h_user_ids))
         ).all()
+        if not sync_lms_users:
+            self.request.response.status_int = 400
+            return {
+                "message": "No users for this grade sync. Can't find any of the provided users"
+            }
+
         sync_lms_users_by_h_userid: dict[str, LMSUser] = {
             lms_user.h_userid: lms_user for lms_user in sync_lms_users
         }

tests/unit/lms/views/dashboard/api/grading_test.py

@@ -11,6 +11,30 @@
     "dashboard_service", "ltia_http_service", "auto_grading_service"
 )
 class TestDashboardGradingViews:
+    def test_create_grading_sync_with_no_lms_users(
+        self,
+        pyramid_request,
+        views,
+        auto_grading_service,
+        dashboard_service,
+        assignment,
+    ):
+        dashboard_service.get_request_assignment.return_value = assignment
+        auto_grading_service.get_in_progress_sync.return_value = None
+        pyramid_request.parsed_params["grades"] = [
+            {"h_userid": "NON EXISTING", "grade": 0.5},
+        ]
+
+        views.create_grading_sync()
+
+        dashboard_service.get_request_assignment.assert_called_once_with(
+            pyramid_request
+        )
+        auto_grading_service.get_in_progress_sync.assert_called_once_with(
+            dashboard_service.get_request_assignment.return_value
+        )
+        pyramid_request.response.status_int = 400
+
     def test_create_grading_sync_with_existing_sync(
         self,
         pyramid_request,