Fix missing null annotation
Fix missing null annotation

OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher