Add migration to create a spree_users table if it doesn't already exist
user
```ruby
class CreateUsers < ActiveRecord::Migration
  def up
    unless table_exists?("spree_users")
      create_table "spree_users", :force => true do |t|
        t.string   "crypted_password",          :limit => 128
        t.string   "salt",                      :limit => 128
        t.string   "email"
        t.string   "remember_token"
        t.string   "remember_token_expires_at"
        t.string   "persistence_token"
        t.string   "single_access_token"
        t.string   "perishable_token"
        t.integer  "login_count",                              :default => 0, :null => false
        t.integer  "failed_login_count",                       :default => 0, :null => false
        t.datetime "last_request_at"
        t.datetime "current_login_at"
        t.datetime "last_login_at"
        t.string   "current_login_ip"
        t.string   "last_login_ip"
        t.string   "login"
        t.integer  "ship_address_id"
        t.integer  "bill_address_id"
        t.datetime "created_at",                                              :null => false
        t.datetime "updated_at",                                              :null => false
        t.string   "openid_identifier"
      end
    end
  end
end
```
assistant