Simplify account model username presence validation spec (#31013)

spec/models/account_spec.rb

@@ -722,11 +722,7 @@ RSpec.describe Account do
   end
 
   describe 'validations' do
-    it 'is invalid without a username' do
-      account = Fabricate.build(:account, username: nil)
-      account.valid?
-      expect(account).to model_have_error_on_field(:username)
-    end
+    it { is_expected.to validate_presence_of(:username) }
 
     it 'squishes the username before validation' do
       account = Fabricate(:account, domain: nil, username: " \u3000bob \t \u00a0 \n ")