Run notify job only when the master service ID is explicitly defined

[davidor]

This is related to the previous PR: #237

We assumed that we could set a default for the master service ID, but after talking with the Porta team, we realized that's not a safe assumption.

This PR changes the configuration setup so it no longer sets a default for the service ID, and also runs notify jobs only when that ID has been set.

[bors[bot]]

:clock1: Waiting for PR status (Github check) to be set, probably by CI. Bors will automatically try to run when all required PR statuses are set.

[davidor]

bors retry

[bors[bot]]

Build succeeded:

• ci/circleci: build