🐛 [open-formulieren/open-forms#3536] Manually parse product amount field

Using zod parse or parseSafe (to not crash) can't properly handle invalid
amount values for a single product. This happens when clearing the
input and causing a crash of the entire form.

Since we only want to pass down the (valid) data for subsequent product
selections, we need some additional robustness on data that is
potentially invalid.

This is to hold us over until we can properly do this in typescript
and get type safety through a compiler instead of chasing after
the facts through propTypes.

src/components/appointments/steps/ChooseProductStep.js

@@ -24,7 +24,7 @@ const productSchema = z
   .array(
     z.object({
       productId: z.string(),
-      amount: z.number().int().gte(1).finite(),
+      amount: z.union([z.number().int().gte(1).finite(), z.literal('').transform(() => 0)]),
     })
   )
   .nonempty();
@@ -35,7 +35,7 @@ const chooseMultiProductSchema = z.object({products: productSchema});
 const ChooseProductStepFields = ({values: {products = []}, validateForm}) => {
   const intl = useIntl();
   const {supportsMultipleProducts} = useContext(AppointmentConfigContext);
-  products = productSchema.parse(products);
+  products = products.map(product => ({...product, amount: Number(product.amount || '0')}));
 
   /**
    * Decorate an arrayHelper callback to invoke the validate function.