Don't attempt to publish build artifacts in PR if they do not exist

[NickGerleman]

The GitHub view of AZP failures shows the most recent failure. This almost always shows the incorrect root cause error below:

This is because we unconditionally try to upload build artifacts, even if an earlier step to produce them fails. We should avoid attempting to publish these artifacts in the case where they might not exist.

[chrisglein]

Could make the publish a warning as a simple way to make this clearer.