fix(ci): keep a main push green when the release run publishes nothing

[Mearman]

Any push to main whose commits all fall outside every package path finishes red — while every one of its ten real jobs passes and nothing in the run reports a failure.

Two recent examples, both with failed_jobs=0:

run head	released	jobs	conclusion
151e36f9	12 packages	42	success
2788390e	nothing	10	failure
75956c0e	nothing	10	failure

Cause

The three post-release fan-out jobs build their matrix from the release job's outputs:

matrix:
  include: ${{ fromJSON(needs.release.outputs.github-packages-matrix) }}

Those outputs are [] when the orchestrator finds nothing to release. matrix: include: [] is not an empty matrix that skips the job — it is an invalid matrix. The job never instantiates, so it appears nowhere in the job list, and the run concludes failure with no failed job to point at.

The release job's own comment asserted the opposite ("leaves the consuming job's matrix empty and the job skipped"), which is presumably why it was never guarded. That comment is corrected here rather than left to mislead the next reader.

Fix

Each consuming job now tests its own matrix output for emptiness in its if, alongside the existing branch/event conditions.

Verifying this

This PR is itself the test case: it only touches .github/, so merging it produces a main push that releases nothing — precisely the condition that currently fails. If the post-merge run is green, the fix holds.

Pre-existing, not a regression from the recent CI work — it was simply latent until a main push happened to release nothing.