DEV: Fix flaky table builder system spec (#25272)

Why this change?

The `Table Builder when editing a table when cancelling table creation should close the modal if there are no changes made` system
test in `spec/system/table_builder_spec.rb` was flaky. It turns out that
when the modal is opened, we have to load some JS/CSS files. While that
is happening, the modal is actually not functional and clicking stuff
in the footer can actually result in an error. In this case, the
`interceptCloseModal` calls the private `_hasChanges` function which
then calls `this.spreadsheet.getHeaders()`. When stuff is still loading,
`this.spreadsheet` has not been set. As a result we get the following
error:

```
Cannot read properties of null (reading 'getHeaders')
```

What does this change do?

Why stuff is loading, we will now hide the footer in the modal.

app/assets/javascripts/discourse/app/components/modal/spreadsheet-editor.gjs

@@ -328,6 +328,7 @@ export default class SpreadsheetEditor extends Component {
       </:body>
 
       <:footer>
+        {{#unless this.loading}}
           <div class="primary-actions">
             <DButton
               @label={{this.modalAttributes.insertTable.title}}
@@ -380,6 +381,8 @@ export default class SpreadsheetEditor extends Component {
               </ul>
             </DTooltip>
           </div>
+        {{/unless}}
+
       </:footer>
     </DModal>
   </template>