Adapt to CurrentUser#getUserName changes

[1] modifies CurrentUser#getUserName to return Optional<String>
instead of nullable String.

This fixes having Optional[user_name] in the download_ref name.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/153310

Change-Id: Ic99f69b293b273a65b911fa4b19d4399172c473f