Creating relationship within 2 SP list with null value (one to many)

Hi Experts,

I am creating a PowerBi that is conecting  to 3 SharePoint lists, however I am facing an issue when I try to creat a relationship due to a "null" value.

List1: MD

List2: Assignmet

List3: Rotation Dates

I merged the infromation from the "MD" and  "Assignmet" and create a new table called "MergeMD&Assignmet". 

Then I tried to create a relationship between the new table "MergeMD&Assignmet" and the table of  "Rotation Dates"  as one-to-many,  however I have not been able to since one of my fields in "MergeMD&Assignmet" has a "null" value ".

Is it possible to have this relationship, even though one of my values is "null"?

I am creating a process where the user will enter into the SharePoint and fill the information found in the First SP list. After a month, the user will enter again and fill the information found in the Second Sp list, and this is when the IDRotation will be created. It will not be creted at the beginning, and due to this, is the "null" value in the column.

Thank you in advance!