fix(templates): optional chaining bug (#8268)

Another one that ts non-strict mode is not catching 🙃🫠


https://github.com/user-attachments/assets/7cf78db9-4181-4a2d-9a8e-cdf3dd4f1951

Author: GermanJablo

templates/website/src/fields/slug/SlugComponent.tsx

@@ -38,7 +38,7 @@ export const SlugComponent: React.FC<SlugComponentProps> = ({
   })
 
   const fieldToUseValue = useFormFields(([fields, dispatch]) => {
-    return fields[fieldToUse].value as string
+    return fields[fieldToUse]?.value as string
   })
 
   useEffect(() => {