readall: use named to check if ARGV is empty.

Otherwise e.g. --debug will be interpreted as a (bad) tap argument.

Closes Homebrew/homebrew#38569.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/readall.rb

@@ -31,7 +31,7 @@ module Homebrew
     end
 
     formulae = []
-    if ARGV.empty?
+    if ARGV.named.empty?
       formulae = Formula.names
     else
       user, repo = tap_args