Fix: set on_behalf to null in the database if the corresponding form field is empty.

app/views/staff.py

@@ -59,7 +59,7 @@ def create_project():
             title=form.title.data,
             description=form.description.data,
             proposer=current_user.username,
-            on_behalf=form.on_behalf.data,
+            on_behalf=form.on_behalf.data if form.on_behalf.data != "" else None,
         )
         db.session.add(project)
         db.session.commit()