All ETs should have a result expression or response variable by default

[vringar]

Is your feature request related to a problem? Please describe.

As a first time user, I expect my call to connectors to show something in operate

Describe the solution you'd like

Every ET should have either a result expression or a result variable.

Describe alternatives you've considered

(Perhaps additionally): Add a modeler lint to ensure that one or the other is always present.

Additional context

Came out of the discussion around individual task testing with @nikku and @bkenez