fix: guard against missing input key in Attributes::extractSwatchType

Attributes configured with `frontend_input` instead of `input` (e.g.
the `rrp` and `colour` samples) have no `input` key in their config
array. PHP 8 raises "Undefined array key" at the bare array access on
line 142. Replace with `$attributeConfig['input'] ?? ''` so the swatch
check degrades gracefully for those attributes.

Author: chevli

Component/Attributes.php

@@ -139,7 +139,7 @@ private function handleExistingAttribute(mixed $attributeCode, array $attributeA
      */
     private function extractSwatchType(array &$attributeConfig): string|false
     {
-        if (!in_array($attributeConfig['input'], ['swatch_text', 'swatch_visual'])) {
+        if (!in_array($attributeConfig['input'] ?? '', ['swatch_text', 'swatch_visual'])) {
             return false;
         }