upgrade: put exit statements in both conditional branches

This check was only occurring in one branch of the conditional.
As a result, if you ran `brew upgrade` with no args and there were no packages to upgrade,
the nonsensical message

"==> Upgrading 0 outdated package, with result:"

would be printed.

Closes Homebrew/homebrew#21316.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/upgrade.rb

@@ -13,6 +13,7 @@ module Homebrew extend self
 
     if ARGV.named.empty?
       outdated = Homebrew.outdated_brews
+      exit 0 if outdated.empty?
     else
       outdated = ARGV.formulae.select do |f|
         if f.installed?