Simplify basic presence validations (#29664)

spec/models/domain_allow_spec.rb

@@ -4,11 +4,7 @@ require 'rails_helper'
 
 RSpec.describe DomainAllow do
   describe 'Validations' do
-    it 'is invalid without a domain' do
-      domain_allow = Fabricate.build(:domain_allow, domain: nil)
-      domain_allow.valid?
-      expect(domain_allow).to model_have_error_on_field(:domain)
-    end
+    it { is_expected.to validate_presence_of(:domain) }
 
     it 'is invalid if the same normalized domain already exists' do
       _domain_allow = Fabricate(:domain_allow, domain: 'にゃん')