trivial: fu-util: correct an assertion when no remotes configured
trivial: fu-util: correct an assertion when no remotes configured

```
(fwupdmgr:185983): FuMain-CRITICAL **: 15:20:57.044: fu_util_time_to_str: assertion 'tmp != 0' failed
```

hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd