cmd/uninstall: handle unparsed arguments.

If `Homebrew.args` hasn't yet been populated then `named` will return
`nil`. Instead, ensure it is an array before we try to `join` it.

This is pretty much only a test-time problem but has been causing flaky
builds.

Library/Homebrew/cmd/uninstall.rb

@@ -130,7 +130,7 @@ module Homebrew
     protected
 
     def sample_command
-      "brew uninstall --ignore-dependencies #{Homebrew.args.named.join(" ")}"
+      "brew uninstall --ignore-dependencies #{Array(Homebrew.args.named).join(" ")}"
     end
 
     def are_required_by_deps