Fix ssh-completions when there's no includes

[danmichaelo]

The ssh-completion didn't work when I first tried to source it. Turned out that reduce failed when $includes was an empty list:

$includes | par-each {|p| $p | open --raw | process } | reduce {|it| merge deep $it --strategy=append }

The change proposed in this PR fixes the problem so the completion also works if there are no includes in the ssh config files. I'm a total nushell beginner though, so I'm not sure if it's the best or the most idiomatic way to solve the problem :)

[fdncred]

Thanks