Fix Slack notifications when upgrading

Make the presence check more robust to only allow
values that make sense.

app/models/project_services/chat_notification_service.rb

@@ -84,10 +84,11 @@ class ChatNotificationService < Service
 
     event_type = data[:event_type] || object_kind
 
-    channel_names = get_channel_field(event_type).presence || channel
+    channel_names = get_channel_field(event_type).presence || channel.presence
+    channels = channel_names&.split(',')&.map(&:strip)
 
     opts = {}
-    opts[:channel] = channel_names.split(',').map(&:strip) if channel_names
+    opts[:channel] = channels if channels.present?
     opts[:username] = username if username
 
     return false unless notify(message, opts)

changelogs/unreleased/212073-slack-notifications-stop-working-after-updating-gitlab.yml

+---
+title: Fix Slack notifications when upgrading from old GitLab versions
+merge_request: 29111
+author:
+type: fixed

spec/models/project_services/chat_notification_service_spec.rb

@@ -75,6 +75,30 @@ describe ChatNotificationService do
       end
     end
 
+    context 'with "channel" property' do
+      before do
+        allow(chat_service).to receive(:channel).and_return(channel)
+      end
+
+      context 'empty string' do
+        let(:channel) { '' }
+
+        it 'does not include the channel' do
+          expect(chat_service).to receive(:notify).with(any_args, hash_excluding(:channel)).and_return(true)
+          expect(chat_service.execute(data)).to be(true)
+        end
+      end
+
+      context 'empty spaces' do
+        let(:channel) { '  ' }
+
+        it 'does not include the channel' do
+          expect(chat_service).to receive(:notify).with(any_args, hash_excluding(:channel)).and_return(true)
+          expect(chat_service.execute(data)).to be(true)
+        end
+      end
+    end
+
     shared_examples 'with channel specified' do |channel, expected_channels|
       before do
         allow(chat_service).to receive(:push_channel).and_return(channel)