Fix bug when there is no genuine edges or build edges

When I refactored the get_truths_exatrkx function, I forgot to include the cases when

• there are no true edges
• there are no edges built by the kNN (this case should not be possible though).

I handled these 2 cases using if/elif conditions

This bug was first revealed during the hands-on session with Gabriel and Benjamin.

@fgiasemi : please just hit the merge button if you think it is ok or tell me when you want to merge it.