little refactor

buisnesslogic/src/main/java/com/plannaplan/services/GroupService.java

@@ -42,14 +42,13 @@ public class GroupService {
         return this.repo.findById(id);
     }
 
-    public Long findNotExistingGroup(List<Long> ids) {
-        Long response = Long.MIN_VALUE;
+    public Optional<Long> findNotExistingGroup(List<Long> ids) {
         for (Long oneId : ids) {
             if (this.repo.existsById(oneId) == false) {
-                response = oneId;
+                return Optional.of(oneId);
             }
         }
-        return response;
+        return Optional.empty();
 
     }
 }