presto-main/src/main/java/io/prestosql/operator/unnest/MapUnnester.java/MapUnnester/appendNulls
class MapUnnester:    @Override
    protected void appendNulls(int count)
    {
        for (int i = 0; i < count; i++) {
            getBlockBuilder(0).appendNull();
            getBlockBuilder(1).appendNull();
        }
    }