cmd/doctor: fix Rubocop warnings.

Library/Homebrew/cmd/doctor.rb

@@ -40,7 +40,7 @@ module Homebrew
       end
 
       out = checks.send(method)
-      unless out.nil? || out.empty?
+      next if out.nil? || out.empty?
       if first_warning
         $stderr.puts <<-EOS.undent
             #{Tty.white}Please note that these warnings are just used to help the Homebrew maintainers
@@ -54,7 +54,6 @@ module Homebrew
       Homebrew.failed = true
       first_warning = false
     end
-    end
 
     puts "Your system is ready to brew." unless Homebrew.failed?
   end