fix: align action buttons in empty bundles on Bitstreams tab (DSpace#5115)

The bundle row's action column was missing the column size class that
the header and bitstream rows use. Without it, the action buttons
don't align vertically when a bundle has no bitstreams.

Add columnSizes.columns[3].buildClasses() to the bundle-row action td,
matching the pattern used in the header th and bitstream-row td elements.

Author: lakshmisravya123

src/app/item-page/edit-item-page/item-bitstreams/item-edit-bitstream-bundle/item-edit-bitstream-bundle.component.html

@@ -34,7 +34,7 @@
               <th id="{{ bundleName }}" class="row-element" colspan="3" scope="colgroup">
                 {{'item.edit.bitstreams.bundle.name' | translate:{ name: bundleName } }}
               </th>
-              <td class="text-center row-element">
+              <td class="text-center row-element {{ columnSizes.columns[3].buildClasses() }}">
                 <div class="btn-group">
                   <button [routerLink]="[itemPageRoute, 'bitstreams', 'new']"
                     [queryParams]="{bundle: bundle.id}"