feat(recipes): send null instead of undefined for blank serves/cookTimeMin

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/(app)/recipes/[id]/edit/+page.server.ts

@@ -72,8 +72,8 @@ export const actions: Actions = {
 			params: { path: { id: params.id } },
 			body: {
 				name: name.trim(),
-				serves: serves ? Number(serves) || undefined : undefined,
-				cookTimeMin: cookTimeMin ? Number(cookTimeMin) || undefined : undefined,
+				serves: serves ? Number(serves) || null : null,
+				cookTimeMin: cookTimeMin ? Number(cookTimeMin) || null : null,
 				effort,
 				heroImageUrl,
 				ingredients: (parsedIngredients as { name: string; quantity: string; unit: string }[])