Only attempt to restore Pages if we have a backup

If a GitHub Enterprise appliance doesn't have any Pages generated or Pages have always been disabled on the appliance, there won't be a "Pages" directory to backup resulting in no "pages" directory in the backup when using `rsync`.

This causes a problem when restoring as  `ghe-restore-pages-rsync` doesn't take this into account and fails with:

```
Starting rsync restore of ghe.vm from snapshot 20140821T133259
Connect ghe.vm OK (v11.10.343)
Enabling maintenance mode on ghe.vm ...
Restoring Git repositories ...
Restoring GitHub Pages ...
rsync: link_stat "/path/to/backup-utils/data/20140821T133259/pages/." failed: No such file or directory (2)
rsync error: some files could not be transferred (code 23) at /SourceCache/rsync/rsync-42/rsync/main.c(992) [sender=2.6.9]
```

This PR corrects this so a restore of Pages over rsync will only be attempted if there is a backup to restore.