Billing - Fix (#18135)

Context : https://github.com/twentyhq/private-issues/issues/425
Try to fix https://github.com/twentyhq/core-team-issues/issues/2158

---------

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

packages/twenty-server/src/engine/core-modules/billing/services/billing-subscription.service.ts

@@ -151,7 +151,7 @@ export class BillingSubscriptionService {
       workspaceId,
     });
 
-    if (subscriptionToCancel) {
+    if (isDefined(subscriptionToCancel)) {
       await this.stripeSubscriptionService.cancelSubscription(
         subscriptionToCancel.stripeSubscriptionId,
       );