Fix ssh-completions when there's no includes (#1065)

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

```nushell
$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 :)

custom-completions/ssh/ssh-completions.nu

@@ -89,7 +89,7 @@ def "nu-complete ssh-host" [] {
         $r.includes = $r.includes | each {|f| $folder | path join $f }
         $r
     } | reduce {|it| merge deep $it --strategy=append }
-    let hosts = $first_result.hosts
+
     let $includes: list<string> = $first_result.includes | each {|f|
         if '*' in $f {
             glob $f
@@ -99,8 +99,11 @@ def "nu-complete ssh-host" [] {
     } | flatten
 
     # Process include files
-    let second_result = $includes | par-each {|p| $p | open --raw | process } | reduce {|it| merge deep $it --strategy=append }
-    # We don't further process "Include" lines in these secondary files.
-    let hosts = $hosts ++ $second_result.hosts
+    let included_hosts = (if ($includes | is-empty) { [] } else {
+        let second_result = $includes | par-each {|p| $p | open --raw | process } | reduce {|it| merge deep $it --strategy=append }
+        $second_result.hosts
+    })
+
+    let hosts = $first_result.hosts ++ $included_hosts
     $hosts | each { {value: $in.name, description: $in.addr } }
 }