John Parejko discovered that lsst.verify.tasks.TimingMetricTask causes an empty quantum graph if the task being timed was never run. Investigate whether this is related to the introduction of optional inputs and/or NoWorkFound in the Middleware framework. Note that, in his scenario, the pipeline was structurally incapable of generating the expected metadata, while the "return if irrelevant" policy was intended to handle pipeline failures.

Thanks for offering to review this, Jim Bosch! Note that there is one PR, ap_pipe#125, that didn't get picked up by Jira.

Also, FTR: I had to drop the "Remove MetricTask.runQuantum" bullet after a Slack discussion revealed that it's still needed for None output support.

Looks great!  Nice simplifications, and as usual your commit discipline and messages are much appreciated.  My only comment (on the ip_diffim PR) is perhaps the smallest, most trivial comment it is possible to make on a PR.