cmd/doctor: fix Rubocop warnings.

Library/Homebrew/cmd/doctor.rb

@@ -40,20 +40,19 @@ module Homebrew
       end
 
       out = checks.send(method)
-      unless out.nil? || out.empty?
-        if first_warning
-          $stderr.puts <<-EOS.undent
+      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
             with debugging if you file an issue. If everything you use Homebrew for is
             working fine: please don't worry and just ignore them. Thanks!#{Tty.reset}
-          EOS
-        end
-
-        $stderr.puts
-        opoo out
-        Homebrew.failed = true
-        first_warning = false
+        EOS
       end
+
+      $stderr.puts
+      opoo out
+      Homebrew.failed = true
+      first_warning = false
     end
 
     puts "Your system is ready to brew." unless Homebrew.failed?