[buildbot] Set build status as passed when a target has no builds

Summary:
When no builds are run on a diff, Harbormaster never sees a success message.
See D8507 as a recent example. This behavior was not noticable prior to D8483.

Depends on D8508

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8509