FIX: Don't raise an exception if we can't update the user on demotion

This is causing issues when purging old users, if they are set up in the
exact condition where they will be demoted into another group, but also
do not have a primary email.

lib/promotion.rb

@@ -126,8 +126,8 @@ class Promotion
     # Then consider the group locked level
     user_group_granted_trust_level = user.group_granted_trust_level
 
-    unless user_group_granted_trust_level.blank?
-      return user.update!(
+    if user_group_granted_trust_level.present?
+      return user.update(
         trust_level: user_group_granted_trust_level
       )
     end

spec/services/user_destroyer_spec.rb

@@ -390,6 +390,17 @@ describe UserDestroyer do
           d.destroy(user)
         }.to change { User.count }.by(-1)
       end
+
+      it 'can delete the user if they were to fall into another trust level and have no email' do
+        g2 = Fabricate(:group, grant_trust_level: 1)
+        g2.add(user)
+
+        UserEmail.where(user: user).delete_all
+        user.reload
+        expect {
+          UserDestroyer.new(admin).destroy(user)
+        }.to change { User.count }.by(-1)
+      end
     end
 
     context 'user has staff action logs' do