DEV: Fix incorrect SyncTimerableIdTopicId migration (#34837)

Follow-up to eeeb7d302f

When `timerable_id` is `null`, `timerable_id != topic_id` will evaluate
to `null` so the row isn't updated.

db/migrate/20250902072941_sync_timerable_id_topic_id.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+
 class SyncTimerableIdTopicId < ActiveRecord::Migration[8.0]
   disable_ddl_transaction!
 
@@ -9,7 +10,7 @@ class SyncTimerableIdTopicId < ActiveRecord::Migration[8.0]
     (min_id..max_id).step(batch_size) { |start_id| execute <<~SQL.squish } if min_id && max_id
         UPDATE topic_timers
         SET timerable_id = topic_id
-        WHERE id >= #{start_id} AND id < #{start_id + batch_size} AND timerable_id != topic_id
+        WHERE id >= #{start_id} AND id < #{start_id + batch_size} AND (timerable_id IS NULL OR timerable_id <> topic_id)
       SQL
   end
 

spec/db/migrate/20250902072941_sync_timerable_id_topic_id_spec.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require Rails.root.join("db/migrate/20250902072941_sync_timerable_id_topic_id.rb")
+
+RSpec.describe SyncTimerableIdTopicId do
+  before do
+    @original_verbose = ActiveRecord::Migration.verbose
+    ActiveRecord::Migration.verbose = false
+  end
+
+  after { ActiveRecord::Migration.verbose = @original_verbose }
+
+  it "works" do
+    DB.exec("DROP TRIGGER IF EXISTS topic_timers_topic_id_trigger ON topic_timers")
+    Migration::ColumnDropper.drop_readonly(:topic_timers, :topic_id)
+    DB.exec("ALTER TABLE topic_timers ALTER COLUMN timerable_id DROP NOT NULL")
+
+    DB.exec(
+      "INSERT INTO topic_timers (execute_at, status_type, type, created_at, updated_at, user_id, topic_id)
+      VALUES (NOW() + INTERVAL '60 minutes', 0, 'TopicTimer', NOW(), NOW(), 1, 12345)",
+    )
+
+    # Insert with id `20000` to ensure batching works as expected.
+    DB.exec(
+      "INSERT INTO topic_timers (id, execute_at, status_type, type, created_at, updated_at, user_id, topic_id)
+      VALUES (20000, NOW() + INTERVAL '60 minutes', 0, 'TopicTimer', NOW(), NOW(), 1, 67890)",
+    )
+
+    SyncTimerableIdTopicId.new.up
+
+    row = DB.query("SELECT * FROM topic_timers WHERE topic_id = 12345")[0]
+
+    expect(row.timerable_id).to eq(12_345)
+
+    row = DB.query("SELECT * FROM topic_timers WHERE topic_id = 67890")[0]
+
+    expect(row.timerable_id).to eq(67_890)
+  end
+end