I was building an entity relationship model (MER) from a database for a rual association, it's a work for TABD stuff. But I came across an atypical situation for me, a relationship 0, n for 0.1. As you can see in the figure. 
It turns out that in this database not always a transaction will have a related person (the transaction can be the money exit with payment of accounts for example) and as the foreign key is in the transaction table this would imply in foreign keys null. My question is if this is correct? Foreign keys can be null? If not, why?
I searched in some places before asking here but there was no agreement on the answers.