Avoid error if no log file exists

There are no log files on a freshly provisioned GHE appliance. Consequently, Hubble cannot find any log file and experiences errors such as this one:
```
Started update of git-download.tsv
gzip: /var/log/github-audit.log.1*.gz: No such file or directory
```

Start by tracing the update of git-download.tsv and how Hubble handles /var/log/github-audit.log.1*.gz when no matching log exists. Reproduce the fresh-appliance case, then verify that the update completes without the gzip “No such file or directory” error when no log file is present.

Written by the indexing model from the issue text.