fix(web): make ShopOfferView.effectSummary/requirements required

The server always sends both fields; an optional frontend type only
dodged a transient gap in merchant.store.spec.ts's shopView() fixture
and risked a requirement line silently failing to render if a future
fixture omitted the fields. Add the two fields to that single fixture
instead (an open offer, so null/[] are the honest values) and drop the
now-unnecessary `?? []` tolerance in the template.

apps/web/src/app/features/npc/merchant.store.spec.ts

@@ -89,6 +89,8 @@ function shopView(): ShopView {
         currencyType: 'SILVER',
         price: 12,
         quantity: 1,
+        effectSummary: null,
+        requirements: [],
         unlocked: true,
         affordable: true,
       },