Make GeneratedAttribute work without ActiveRecord

If GeneratedAttribute is used in a project that doesn't require
ActiveRecord, it should still work.

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>

railties/lib/rails/generators/generated_attribute.rb

@@ -65,11 +65,13 @@ def parse(column_definition)
         end
 
         def dangerous_name?(name)
-          ActiveRecord::Base.dangerous_attribute_method?(name)
+          defined?(ActiveRecord::Base) &&
+            ActiveRecord::Base.dangerous_attribute_method?(name)
         end
 
         def valid_type?(type)
           DEFAULT_TYPES.include?(type.to_s) ||
+            !defined?(ActiveRecord::Base) ||
             ActiveRecord::Base.connection.valid_type?(type)
         end