FIX: Error when loading a channel with threading enabled but no threads (#22434)

Without this fix, the following error is raised:

```
ActiveRecord::StatementInvalid:
  PG::SyntaxError: ERROR:  syntax error at or near ")"
  LINE 4:   WHERE thread_id IN ()
```

plugins/chat/app/services/chat/lookup_channel_threads.rb

@@ -84,6 +84,7 @@ module Chat
 
       threads = unread_threads + read_threads
 
+      if threads.present?
         last_replies =
           Chat::Message
             .strict_loading
@@ -102,6 +103,7 @@ module Chat
             .index_by(&:thread_id)
 
         threads.each { |thread| thread.last_reply = last_replies[thread.id] }
+      end
 
       threads
     end

plugins/chat/spec/services/chat/lookup_channel_threads_spec.rb

@@ -10,6 +10,7 @@ RSpec.describe Chat::LookupChannelThreads do
 
     fab!(:current_user) { Fabricate(:user) }
     fab!(:channel) { Fabricate(:chat_channel, threading_enabled: true) }
+    fab!(:channel_with_no_threads) { Fabricate(:chat_channel, threading_enabled: true) }
     fab!(:thread_1) { Fabricate(:chat_thread, channel: channel) }
     fab!(:thread_2) { Fabricate(:chat_thread, channel: channel) }
     fab!(:thread_3) { Fabricate(:chat_thread, channel: channel) }
@@ -32,6 +33,12 @@ RSpec.describe Chat::LookupChannelThreads do
         end
       end
 
+      it "does not return any threads when a channel has no threads" do
+        expect(
+          described_class.call(channel_id: channel_with_no_threads.id, guardian:).threads,
+        ).to eq([])
+      end
+
       context "when all steps pass" do
         before do
           msg_1 =