Rails - Aug 2014 - password validation triggered even I update non-password attribute

I have a User model:

  create_table "users", force: true do |t|
    t.string   "name"
    t.string   "email"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.string   "password_digest"
  end
  add_index "users", ["name"], name:
"index_users_on_name"

Then, I have the following validation:

class User < ActiveRecord::Base
  has_secure_password
  validates :password, length: {minimum: 4}
end

Then, I update in my controller the name and email of a certain user:

   ...
    logger.debug('++++++++ new_values after: '+new_values.inspect)
    if @user.update_attributes(new_values)
      logger.debug('+++++++++++++ SUCCESS')
      ...
   end

(new_values is of type ActionController::Parameters)

My log shows

  ++++++++ new_values after: {"name"=>"1111",
"email"=>"1111"}

However, the update_attributes fails, and the error message says:

  Password is too short (minimum is 4 characters)

What I don't understand is, that I don't supply a new password. Why,
then, is password validation triggered here? And how should I implement
this?

Background for this question:

This code is executed only, when a user wants to edit his profile data
(which, for the time being, consist only of name and email). It is
ensured that only a logged in user can execute this code, and only for
his own data. Therefore, I don't require to enter the password. He had
entered it anyway when logging in.

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