In Ruby on Rails, to not check nil cases with validates numericality, specify allow_nil: true.

class Article < ActiveRecord::Base
  validates :page_view, numericality: true, allow_nil: true
end

Without specifying allow_nil: true, even empty input will result in a validation error saying “xxx must be a number.”

・[Reference]：validation - custom error message for valid numericality of in rails - Stack Overflow

That’s all from the Gemba.