Fix some bugs in the archival queue task executor

[MichaelSnowden]

What changed?
After manual testing, I spotted a few issues in this code.

1. We fail the task if there are zero enabled targets for archival, which might happen if a task is produced and then archival is disabled before the task is executed. In this case, we should continue processing the task, skipping archival but generating the deletion task.
2. We are parsing archival URIs unconditionally here, but we should only parse them for enabled targets. We cannot assume that those URIs will be valid when the corresponding target is disabled.

Why?
Bug fixes

How did you test it?
I modified the unit tests. I also reverted the code changes other than the test code and verified that the tests "URIs are not read for empty targets" and "both history and visibility archival disabled" failed.

Potential risks
None, just bug fixes.

Is hotfix candidate?
No