feat(graphResolvers): return null for cds_year stats

Author: ChiaraZuccaro

src/app/values/graph-resolvers/graph-resolvers.value.ts

@@ -20,7 +20,8 @@ export function GraphResolvers(
       return data ? GraphMapper.toTeachingGraph(data) : null;
     },
     cds_year: () => {
-      throw new Error('Function not implemented.');
+      console.error('Function not implemented.');
+      return null;
     },
   };
 }