Save-Package-Properties should also check artifact list

If running against a specific service directory, we need to be a more specific about how the error case is handled.

[This cloudmachine build failure](https://github.com/Azure/azure-sdk-for-net/pull/48487/checks?check_run_id=38011665959) is because we updated the package names that were output, but didn't update the artifact list to reflect. When the `servicedirectory` is passed to save-package-props, we DO generate those files, but then we immediately filter them away. We filter them away because during a service directory build, we only attempt to build the artifacts that exist _within the owning ci.yml_. When none of the artifacts exist, we end up with nothing to build, and we end up in a situation where we're attempting to reference an unset variable.

@hallipr submitted Azure/azure-sdk-for-net#48494 to resolve the unexpected failure, but we ALSO should be explicit about this issue with the artifact list.