FIX: Exclude users with posts from purge_unactivated query (#12231)

Unactivated users that have posts cannot be deleted so we shouldn't
include them in the initial query to try and purge them. Otherwise we
are just loading up sidekiq with pointless work to be doing every day.

Without this change if there are 201 unactivated users to purge, but the
first 200 have posts, the 201st user will never be deleted even though
it is the only user that doesn't have a post and is actually the one
that should be deleted.

spec/models/user_spec.rb

@@ -1471,6 +1471,7 @@ describe User do
     let!(:unactivated_old) { Fabricate(:user, active: false, created_at: 1.month.ago) }
     let!(:unactivated_old_with_system_pm) { Fabricate(:user, active: false, created_at: 2.months.ago) }
     let!(:unactivated_old_with_human_pm) { Fabricate(:user, active: false, created_at: 2.months.ago) }
+    let!(:unactivated_old_with_post) { Fabricate(:user, active: false, created_at: 1.month.ago) }
 
     before do
       PostCreator.new(Discourse.system_user,
@@ -1486,17 +1487,24 @@ describe User do
                       archetype: Archetype.private_message,
                       target_usernames: [unactivated_old_with_human_pm.username],
       ).create
+
+      PostCreator.new(unactivated_old_with_post,
+                      title: "Test topic from a user",
+                      raw: "This is a sample message"
+      ).create
     end
 
     it 'should only remove old, unactivated users' do
       User.purge_unactivated
-      expect(User.real.all).to match_array([user, unactivated, unactivated_old_with_human_pm])
+      expect(User.real.all).to match_array([user, unactivated, unactivated_old_with_human_pm, unactivated_old_with_post])
     end
 
     it "does nothing if purge_unactivated_users_grace_period_days is 0" do
       SiteSetting.purge_unactivated_users_grace_period_days = 0
       User.purge_unactivated
-      expect(User.real.all).to match_array([user, unactivated, unactivated_old, unactivated_old_with_system_pm, unactivated_old_with_human_pm])
+      expect(User.real.all).to match_array([
+        user, unactivated, unactivated_old, unactivated_old_with_system_pm, unactivated_old_with_human_pm, unactivated_old_with_post
+      ])
     end
   end