Added "empty" message

Modeled on browse-by.component.html

src/app/+collection-page/collection-page.component.html

@@ -52,6 +52,9 @@
                           message="{{'error.recent-submissions' | translate}}"></ds-error>
                 <ds-loading *ngIf="!itemRD || itemRD.isLoading"
                             message="{{'loading.recent-submissions' | translate}}"></ds-loading>
+                <div *ngIf="!itemRD?.isLoading && itemRD?.payload?.page.length === 0" class="alert alert-info w-100" role="alert">
+                  {{'browse.empty' | translate}}
+                </div>
             </ng-container>
         </div>
         <ds-error *ngIf="collectionRD?.hasFailed"