tap: update shallowing logic

Co-Authored-By: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/tap.rb

@@ -55,8 +55,8 @@ module Homebrew
     else
       full_clone = if args.full?
         true
-      elsif args.shallow?.nil?
-        !ENV["CI"]
+      elsif !args.shallow?
+        ENV["CI"].blank?
       else
         !args.shallow?
       end