When encoding zero length String, Jacorb is creating a String of length one with the terminating zero character. 
However, when you switch on the null String encoding (jacorb.interop.null_string_encoding=on), it generates a zero length String, that can break the server ORB which can not unmarshall it.

in CDROutputStream, encoding of a zero length stream:
if (valueLength == 0)
{
    // Blank string; just write size and terminator. Don't need to
    // align here as these methods will do it for us.
    write_long( 1 );
    write_octet( (byte)0 );
}

null string encoding:
if (nullStringEncoding)
{
    write_long(0);
    return;
}