FIX: skip email if blank while syncing SSO attributes. (#19939)

Also, return email blank error in `EmailValidator`  when the email is blank.

spec/models/user_spec.rb

@@ -2573,6 +2573,11 @@ RSpec.describe User do
       expect(User.find(user.id).email).to eq(secondary_email_record.email)
       expect(user.secondary_emails.count).to eq(0)
     end
+
+    it "returns error if email is nil" do
+      user.email = nil
+      expect { user.save! }.to raise_error(ActiveRecord::RecordInvalid)
+    end
   end
 
   describe "set_random_avatar" do