UX: Add render check for multi-select caret icon (#36796)

Prevents a console error when the icon is null due to max item
validation being reached.

[Related meta
topic](https://meta.discourse.org/t/svg-icon-missing-error-with-dropdown-settings-in-theme-object-settings-when-max-input-is-reached/391708/2)

### Before / After

<img width="1516" height="955" alt="image"
src="https://github.com/user-attachments/assets/dad7cee2-93d0-4559-a28b-bffcc4c87b70"
/>

frontend/discourse/select-kit/components/combo-box/combo-box-header.gjs

@@ -42,7 +42,9 @@ export default class ComboBoxHeader extends SingleSelectHeaderComponent {
         />
       {{/if}}
 
-      {{icon this.caretIcon class="caret-icon"}}
+      {{#if this.caretIcon}}
+        {{icon this.caretIcon class="caret-icon"}}
+      {{/if}}
     </div>
   </template>
 }

frontend/discourse/select-kit/components/multi-select/multi-select-header.gjs

@@ -65,8 +65,9 @@ export default class MultiSelectHeader extends SelectKitHeaderComponent {
           @content={{or this.selectedContent this.selectKit.noneItem}}
           @selectKit={{this.selectKit}}
         />
-
-        {{icon this.caretIcon class="caret-icon"}}
+        {{#if this.caretIcon}}
+          {{icon this.caretIcon class="caret-icon"}}
+        {{/if}}
       {{/if}}
     </div>
   </template>