docs vs code discrepancy in Err field

[ahmetb]

https://docs.docker.com/engine/extend/plugins_volume/

this document often cites "Err" field as null, however this SDK often sends empty string ("") if the field is not a non-empty string. Pretty sure it works just fine but this shows:

a. API on the docker-engine side is not well defined nor strict (i.e. relies on Go json decoder and type zero value)
b. this library doesn't exactly do what docs say.