How to ensure unique constraint over multiple columns in Room, even if one of them is null?

I have a Room database in my application with one table containing received and sent messages. Inside of the table, the messages are just differentiated by the phone number, being null for the backend-server (since a server has no phone number) and the phone number of the user for the sent messages. (Entered on app installation, just as Whatsapp.)
To sync the table with the backend, I introduced a new column, containing the backend id of the messages on the server. Since the server seperates sent and received messages (due to different information contained in the tables backend), the id of a sent message and the id of a received message can be equal, only distinguishable by the corresponding phone number. (Either null or own)
So I created a unique constraint over both columns: backend_id & phone number.