Test vectors have an empty string for 'controller' in 'verificationMethod'. Does empty string for “controller” violate the spec? Should we omit “controller” if it is empty?
Relevant sections of did core spec: "A DID document MAY include a controller property to indicate the DID controller(s)." and "When a controller property is present in a DID Document, its value expresses one or more DIDs."
Also addresses https://github.com/decentralized-identity/sidetree/issues/995