presto-spi/src/main/java/io/prestosql/spi/block/VariableWidthBlockBuilder.java/VariableWidthBlockBuilder/appendNull
class VariableWidthBlockBuilder:    @Override
    public BlockBuilder appendNull()
    {
        if (currentEntrySize > 0) {
            throw new IllegalStateException("Current entry must be closed before a null can be written");
        }

        hasNullValue = true;
        entryAdded(0, true);
        return this;
    }