Evaluate the default block only when necessary

Follow up of #28453.

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -21,8 +21,8 @@ def replace(record)
         self.target = record
       end
 
-      def default(record)
-        writer(record) if reader.nil?
+      def default(&block)
+        writer(instance_exec(&block)) if reader.nil?
       end
 
       def reset

activerecord/lib/active_record/associations/builder/belongs_to.rb

@@ -121,7 +121,7 @@ def self.add_touch_callbacks(model, reflection)
 
     def self.add_default_callbacks(model, reflection)
       model.before_validation lambda { |o|
-        o.association(reflection.name).default o.instance_exec(&reflection.options[:default])
+        o.association(reflection.name).default(&reflection.options[:default])
       }
     end