[buildbot] Build targets should default to success when no builds are queued

Summary:
Before this patch, the behavior of build targets without any builds queued is
poorly defined.

This change will be used in a followup diff to ensure a call to /buildDiff
that results in no builds being queued will still succeed.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8508