Fix application rename with CNB lifecycle (#4206)

* Fix application rename with CNB lifecycle

* Simplified new code

---------

Co-authored-by: Ralf Pannemans <ralf.pannemans@sap.com>

Author: pbusko

lib/cloud_controller/diego/lifecycles/app_cnb_lifecycle.rb

@@ -16,7 +16,7 @@ def create_lifecycle_data_model(app)
     end
 
     def valid?
-      !buildpacks.empty?
+      message.is_a?(AppUpdateMessage) || !buildpacks.empty?
     end
 
     def errors

spec/unit/lib/cloud_controller/diego/lifecycles/app_cnb_lifecycle_spec.rb

@@ -102,6 +102,14 @@ module VCAP::CloudController
         it 'invalid' do
           expect(lifecycle.valid?).to be(false)
         end
+
+        context 'during an update' do
+          let(:message) { VCAP::CloudController::AppUpdateMessage.new(request) }
+
+          it 'valid' do
+            expect(lifecycle.valid?).to be(true)
+          end
+        end
       end
 
       context 'with buildpacks' do