Change district_nces_id to be optional, update test (#650)

## Status

-
[Closes](RaspberryPiFoundation/digital-editor-issues#1088)

## What's changed?
 - Change the `district_nces_id` field to be optional

Author: jamiebenstead

app/models/school.rb

@@ -23,7 +23,6 @@ class School < ApplicationRecord
   validates :district_nces_id,
             uniqueness: { conditions: -> { where(rejected_at: nil) }, case_sensitive: false, allow_blank: true, message: I18n.t('validations.school.district_nces_id_exists') },
             format: { with: /\A\d{7}\z/, allow_nil: true, message: I18n.t('validations.school.district_nces_id') },
-            presence: true,
             if: :united_states?
   validates :district_name, presence: true, if: :united_states?
   validates :school_roll_number,

spec/models/school_spec.rb

@@ -214,10 +214,9 @@
       expect(school).to be_valid
     end
 
-    it 'requires district_nces_id for US schools' do
+    it 'does not require district_nces_id for US schools' do
       us_school.district_nces_id = nil
-      expect(us_school).not_to be_valid
-      expect(us_school.errors[:district_nces_id]).to include("can't be blank")
+      expect(us_school).to be_valid
     end
 
     it 'requires district_name for US schools' do