🎨(backend) assign the quota with a walrus operator

Apply a review suggestion to keep the quota lookup and its guard on a
single line in the entitlements endpoint.

src/backend/core/api/viewsets.py

@@ -2468,8 +2468,7 @@ class EntitlementsViewset(viewsets.ViewSet):
                 method = getattr(entitlements_backend, method_name)
                 if callable(method):
                     entitlements[method_name] = method(request.user)
-        quota = entitlements_backend.get_quota(request.user)
-        if quota:
+        if quota := entitlements_backend.get_quota(request.user):
             entitlements["quota"] = quota
         entitlements["context"] = entitlements_backend.get_context(request.user)
         return drf.response.Response(entitlements)