Better handling empty inputs + outputs

[krhubert]

mesg.yml

name: test-service

events:
  foo:

tasks:
  foo:

➜  core git:(benchmark/service) ✗ ./dev-cli service validate ./service
⨯ File 'mesg.yml' is not valid. See documentation: https://docs.mesg.com/guide/service/service-file.html
    * Tasks[foo] is a required field
    * Events[foo] is a required field

The information is not clear. Also there was an issue about handle empty data but I can't find it right now so I recreated this one.

[krhubert]

• extra with validation changes service doesn't have an option to return nothing

name: test-service
tasks:
  foo:
    outputs:
      success:
        data:
          nil:
            type: String

⨯ File 'mesg.yml' is not valid. See documentation: https://docs.mesg.com/guide/service/service-file.html
    * Tasks[foo].Outputs[success].Data is a required field