Make serialized workflow compatible with schema

[fjtirado]

Before the PR, the file resulting of executing this code

Workflow workflow = new Workflow("HelloWorld", "Hello World", "1.0", Arrays.asList(
             new InjectState(START_STATE, Type.INJECT).withData(new ObjectMapper().createObjectNode().put("response","Hello World!!!")).withEnd(new End())))
             .withStart(new Start().withStateName(START_STATE));
try (Writer writer = new FileWriter("helloworld.json")) {
        new JsonObjectMapper().writeValue(writer, workflow);
        } catch (IOException io) {
            throw new UncheckedIOException(io);
        }

was

{
  "id" : "HelloWorld",
  "name" : "HelloWorld",
  "version" : "1_0",
  "start" : "inject_0",
  "expressionLang" : "jq",
  "events" : [ ],
  "functions" : [ ],
  "retries" : [ ],
  "errors" : [ ],
  "secrets" : [ ],
  "states" : [ {
    "data" : {
      "response" : "Hello World!!!"
    },
    "usedForCompensation" : false,
    "name" : "inject_0",
    "type" : "inject",
    "end" : true,
    "onErrors" : [ ]
  } ]
}

which is not compatible with json schema (becasuse, for example, events, functions, retries, errores, secrets should be not present or has at least 1 element)

After the change, the result is

{
  "id" : "HelloWorld",
  "name" : "HelloWorld",
  "version" : "1_0",
  "start" : "inject_0",
  "expressionLang" : "jq",
  "states" : [ {
    "data" : {
      "response" : "Hello World!!!"
    },
    "usedForCompensation" : false,
    "name" : "inject_0",
    "type" : "inject",
    "end" : true
  } ]
}

which is schema compatible

[fjtirado]

@ricardozanini Can we please take a look to this quick (and so needed for editor) fix?