chore: Ignore notification when assignee is nil (#11105)

The issue was that when a team change results in an assignee being set
to nil, the system was still trying to create a notification about the
assignment change, but there was no assignee to notify, causing
potential issues in the notification system.

spec/listeners/notification_listener_spec.rb

@@ -200,4 +200,28 @@ describe NotificationListener do
       end
     end
   end
+
+  describe 'assignee_changed' do
+    let(:event_name) { :'conversation.assignee_changed' }
+
+    context 'when notifiable_assignee_change is true but assignee is nil' do
+      it 'does not create a notification' do
+        conversation_with_nil_assignee = create(:conversation, account: account, inbox: inbox, assignee: nil)
+
+        notification_builder_mock = instance_double(NotificationBuilder)
+        allow(NotificationBuilder).to receive(:new).and_return(notification_builder_mock)
+
+        event = Events::Base.new(
+          event_name,
+          Time.zone.now,
+          conversation: conversation_with_nil_assignee,
+          data: { notifiable_assignee_change: true }
+        )
+
+        expect(notification_builder_mock).not_to receive(:perform)
+
+        listener.assignee_changed(event)
+      end
+    end
+  end
 end