presto-main/src/main/java/io/prestosql/operator/scalar/EmptyMapConstructor.java/EmptyMapConstructor/EmptyMapConstructor
class EmptyMapConstructor:    public EmptyMapConstructor(@TypeParameter("map(unknown,unknown)") Type mapType)
    {
        BlockBuilder mapBlockBuilder = mapType.createBlockBuilder(null, 1);
        mapBlockBuilder.beginBlockEntry();
        mapBlockBuilder.closeEntry();
        emptyMap = ((MapType) mapType).getObject(mapBlockBuilder.build(), 0);
    }