attributePrefix cannot be empty string

[davemevans]

Currently our project uses version 1.0.11, and we set attributePrefix to ''.

We are looking at upgrading to the latest version, but this will not allow attributePrefix to be an empty string since empty strings are falsy and therefore config.attributePrefix || "_" returns "_".

It would be great if x2js explicitly checked for undefined for this option rather than falsy-ness.