Show "Never" when instance has never been used

[lorumic]

Done

• Show "Never" when instance has never been used instead of "Jan 1, 1970 ..."

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @lorumic or @edlerd for access.
   • With a local copy of this branch, run as described here.
2. Perform the following QA steps:
   • Go to the detail page of an instance that was never used, e.g. this
   • Check that "Last used" says "Never" instead of "Jan 1, 1970"

[webteam-app]

Demo starting at https://lxd-ui-539.demos.haus

[edlerd]

Nice, thanks for fixing this.