module Cdx
  module Admin
    class UsersController < ResourceController

    end
  end
end

module Cdx
  module Admin
    class UsersController < ResourceController

      before_action :update_without_password, only: :update

      private

      def update_without_password
        if params[:user][:password].blank?
          params[:user].delete('password')
          params[:user].delete('password_confirmation')
        end
      end

    end
  end
end

Allow update user without password
Allow update user without password
