fix on legend when entry does not have a continent associated
fix on legend when entry does not have a continent associated

OurWorldInData/owid-grapher,owid/owid-grapher,OurWorldInData/our-world-in-data-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,OurWorldInData/our-world-in-data-grapher,owid/owid-grapher,OurWorldInData/our-world-in-data-grapher,OurWorldInData/our-world-in-data-grapher