SWF Viewer : Fix issue with empty Json

When attempting to parse an empty Json file for the first time, this causes error /NPE, if instead of this a valid Json is parsed and then an empty Json file gets parsed this issue does not happen. It is because a marshaller instance has not been initialized. This causes issues when integrating with VSCode

NOTE: 

• It's necessary to identify when creating a new workflow (the tooling uses empty content), or when the json is "empty" or invalid, which means it's about error handling instead.