[#52118601] Fix quoted variable lint warning

Use `inline_template()` to render the FQDN fact to a string and then test
whether it is empty. Extension of the previous commit. Fixes the following
lint warning:

manifests/init.pp - WARNING: string containing only a variable on line 21

I did look at using `empty()` from puppetlabs-stdlib. But even that raises
the same lint problems when you try to pass the FQDN variable in.