feat(AouRelation2TiersController): create with default relation properties if none are given

AoU-Admin/src/main/java/ch/unige/aou/controller/AouRelation2TiersController.java

@@ -83,6 +83,10 @@ public abstract class AouRelation2TiersController<T extends Resource<T>, V exten
     final T parentItem = this.itemService.findOne(parentid);
     final V childItem = this.childItemService.findOne(id);
 
+    if (joinItem == null) {
+      joinItem = this.relationItemService.getDefaultJoinResource();
+    }
+
     if (!this.relationItemService.relationExists(parentItem, childItem)) {
       D childDTO = this.relationItemService.saveRelation(parentItem, childItem, joinItem);
       this.addLinks(parentid, childDTO);