when governed_app_vertex is requested it is no loner None

[Christian-B]

This is a small change mainly designed to show that a change in the Typing branches is not needed.

In that pr there are two method to get the app vertex

governed_app_vertex which could return a None
_governed_app_vertex which never returns a None

This pr shows that governed_app_vertex can just not return None

If someone does need to check if the app vertex is not yet set we should add a has_app_vertex method.

tested by:
http://apollo.cs.man.ac.uk:8080/blue/organizations/jenkins/Integration%20Tests/detail/governed_app_vertex_never_none/1/pipeline