presto-spi/src/main/java/io/prestosql/spi/block/MapBlockBuilder.java/MapBlockBuilder/appendNull
class MapBlockBuilder:    @Override
    public BlockBuilder appendNull()
    {
        if (currentEntryOpened) {
            throw new IllegalStateException("Current entry must be closed before a null can be written");
        }

        entryAdded(true);
        return this;
    }