Simplify basic presence validations (#29664)

spec/models/user_spec.rb

@@ -32,11 +32,7 @@ RSpec.describe User do
   end
 
   describe 'validations' do
-    it 'is invalid without an account' do
-      user = Fabricate.build(:user, account: nil)
-      user.valid?
-      expect(user).to model_have_error_on_field(:account)
-    end
+    it { is_expected.to belong_to(:account).required }
 
     it 'is invalid without a valid email' do
       user = Fabricate.build(:user, email: 'john@')