Filter ungraded colles to avoid unexecepted behaviors

app/services/grade_service.ts

@@ -44,6 +44,7 @@ export class GradeService {
       .where('studentId', userId)
       .where('date', '>=', startDate.toJSDate())
       .where('date', '<', endDate.toJSDate())
+      .whereNotNull('garde')
       .preload('subject')
       .orderBy('date', 'asc')
   }
@@ -53,6 +54,7 @@ export class GradeService {
       .where('studentId', userId)
       .where('subjectId', subjectId)
       .where('date', '<', beforeDate.toJSDate())
+      .whereNotNull('garde')
     return colles.length ? this.calculateAverage(colles) : 0
   }