Fix issue with the Details screen overlay where a null thumbnail change would be sent even though the thumbnail is not editable (RM#3613).

If the user has effective write permission to links (which, eg. an admin should unless the asset is in workflow), the thumbnail would be deleted
in this situation (generally speaking this is when live with workflow applied, where EES restricts editing of the thumbnail but permission to do
so still exists underneath). In cases where there is no write permission (eg. in workflow) it may appear to be deleted but it would actually not
be because the link delete would be denied by Matrix.

Reason: the code was expecting the wrapper "#ees_thumbnailPicker" to be non-existent when thumbnail not editable - but it seems to stick around
although not with the asset picker inside it. So handling either way, either non-existent wrapper or wrapper not having an asset picker
(.ees_assetPicker) inside.