plist_caveats: ensure it ending with newline

Closes Homebrew/homebrew#49374.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/caveats.rb

@@ -212,6 +212,6 @@ class Caveats
 
       s << "" << "WARNING: launchctl will fail when run under tmux." if ENV["TMUX"]
     end
-    s.join("\n") unless s.empty?
+    s.join("\n") + "\n" unless s.empty?
   end
 end