prevent error on new M2M item

This prevent the error commented on this issue :#470 (**Adding item through many-to-many UI throws error**)

The problem with this issue was in fact: ee3ed41b1b953cf5fc45939676b9b19e8d7a921e commit.

I did not keep in mind that new item from M2M can be new items without any id, so if there's not id, not need to compare whether exists or not.