Add nil guard to ProcessModel.run_action_user

* prevents exceptions "docker? delegated to app, but app is nil" when listing large number of processes on a live system

Author: jochenehret

app/models/runtime/process_model.rb

@@ -384,6 +384,7 @@ def started_command
 
     def run_action_user
       return user if user.present?
+      return nil if app.nil?
 
       docker? ? docker_run_action_user : AppModel::DEFAULT_CONTAINER_USER
     end

spec/unit/models/runtime/process_model_spec.rb

@@ -747,6 +747,17 @@ def act_as_cf_admin
           end
         end
       end
+
+      context 'when the associated app is nil' do
+        before do
+          process.app = nil
+        end
+
+        it 'does not raise an error and returns nil' do
+          expect{process.run_action_user}.not_to raise_error
+          expect(process.run_action_user).to be_nil
+        end
+      end
     end
 
     describe '#specified_or_detected_command' do