Hey there @Baptiste_LC,
So there are several posts on this topic, and this thread makes reference to many solutions out there.
The patchy workaround I usually go for when I have clients with the retool DB is to use a bulk update via primary key via an app. I create a query, select my table, copy and paste my csv data into this page that creates a Json and use the output as the items to be updated. This way null values will be updated properly.
Hope this helps!