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-page.component.html

@@ -228,7 +228,7 @@
                   }}</span>
                 }
                 @for (
-                  requirement of offer.requirements ?? [];
+                  requirement of offer.requirements;
                   track requirement.label
                 ) {
                   <span