Fix recalculation problem with null amounts

frontend/src/components/recipes/RecipeDetailPage.tsx

@@ -58,7 +58,8 @@ export default function RecipeDetailPage() {
             ...ingGrp,
             ingredientList: ingGrp.ingredientList.map((ing) => ({
                 ...ing,
-                amount: ing.amount !== undefined ? ing.amount * factor : ing.amount,
+                // ensure only to recalculate the amount of ingredients that actually have an amout...
+                amount: (ing.amount && ing.amount !== undefined) ? ing.amount * factor : undefined,
             }))
         }))