Remove obsolete nil guard

This was added in 3a7a3619f7016c75c3e53e944221bf7f6354a63b to make the
tests pass, but this method is now stubbed out in the tests, so we can
assume $? will be non-nil.

Library/Homebrew/cmd/update.rb

@@ -220,8 +220,8 @@ class Updater
 
   def `(cmd)
     out = super
-    if $? && !$?.success?
+    unless $?.success?
-      $stderr.puts out
+      $stderr.puts(out) unless out.empty?
       raise ErrorDuringExecution.new(cmd)
     end
     ohai(cmd, out) if ARGV.verbose?