Do not populate CurrentAttributes#attributes when not using defaults

When using `CurrentAttributes.attribute` without a default value, the
`#attributes` method should not populate the hash with the attribute.

Closes rails#52901.

Author: rafaelfranca

activesupport/lib/active_support/current_attributes.rb

@@ -95,6 +95,8 @@ class CurrentAttributes
 
     INVALID_ATTRIBUTE_NAMES = [:set, :reset, :resets, :instance, :before_reset, :after_reset, :reset_all, :clear_all] # :nodoc:
 
+    NOT_SET = Object.new.freeze # :nodoc:
+
     class << self
       # Returns singleton instance for this class in this thread. If none exists, one is created.
       def instance
@@ -109,7 +111,7 @@ def instance
       # is a proc or lambda, it will be called whenever an instance is
       # constructed. Otherwise, the value will be duplicated with +#dup+.
       # Default values are re-assigned when the attributes are reset.
-      def attribute(*names, default: nil)
+      def attribute(*names, default: NOT_SET)
         invalid_attribute_names = names.map(&:to_sym) & INVALID_ATTRIBUTE_NAMES
         if invalid_attribute_names.any?
           raise ArgumentError, "Restricted attribute names: #{invalid_attribute_names.join(", ")}"
@@ -221,8 +223,10 @@ def reset
 
     private
       def resolve_defaults
-        defaults.transform_values do |value|
-          Proc === value ? value.call : value.dup
+        defaults.each_with_object({}) do |(key, value), result|
+          if value != NOT_SET
+            result[key] = Proc === value ? value.call : value.dup
+          end
         end
       end
   end

activesupport/test/current_attributes_test.rb

@@ -247,6 +247,10 @@ def after_teardown
     ActiveSupport::IsolatedExecutionState.isolation_level = previous_level
   end
 
+  test "CurrentAttributes doesn't populate #attributes when not using defaults" do
+    assert_equal({ counter_integer: 0, counter_callable: 0 }, Current.attributes)
+  end
+
   test "CurrentAttributes restricted attribute names" do
     assert_raises ArgumentError, match: /Restricted attribute names: reset, set/ do
       class InvalidAttributeNames < ActiveSupport::CurrentAttributes