bugfix: error with reactivity on POST

Related to applications/hubl#569 (closed)

We added some improvements in !381 (merged) to notify nested resources on updates. For example, if I have a form editing users/matthieu, and inside, the field profile.city, I want to notify users/matthieu and profile/matthieu when data changes.

With the same form, when we POST on users and create a profile, we don't want to notify anyone -> this returned an error.

On the following line, we try to get the @id of profile. It works when we edit an existing resource, but not when we are posting a new object on a container: https://git.startinblox.com/framework/sib-core/blob/master/src/libs/store/store.ts#L199