collab: Update billing migration endpoint to work for users without active subscriptions (#29792)

This PR updates the billing migration endpoint to work for users who do
not have an active subscription.

This will allow us to use the endpoint to migrate all users.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -624,7 +624,7 @@ struct MigrateToNewBillingBody {
 #[derive(Debug, Serialize)]
 struct MigrateToNewBillingResponse {
     /// The ID of the subscription that was canceled.
-    canceled_subscription_id: String,
+    canceled_subscription_id: Option<String>,
 }
 
 async fn migrate_to_new_billing(
@@ -650,15 +650,9 @@ async fn migrate_to_new_billing(
         .get_active_billing_subscriptions(HashSet::from_iter([user.id]))
         .await?;
 
-    let Some((_billing_customer, billing_subscription)) =
+    let canceled_subscription_id = if let Some((_billing_customer, billing_subscription)) =
         old_billing_subscriptions_by_user.get(&user.id)
-    else {
-        return Err(Error::http(
-            StatusCode::NOT_FOUND,
-            "No active billing subscriptions to migrate".into(),
-        ));
-    };
-
+    {
         let stripe_subscription_id = billing_subscription
             .stripe_subscription_id
             .parse::<stripe::SubscriptionId>()
@@ -674,6 +668,11 @@ async fn migrate_to_new_billing(
         )
         .await?;
 
+        Some(stripe_subscription_id)
+    } else {
+        None
+    };
+
     let feature_flags = app.db.list_feature_flags().await?;
 
     for feature_flag in ["new-billing", "assistant2"] {
@@ -691,7 +690,8 @@ async fn migrate_to_new_billing(
     }
 
     Ok(Json(MigrateToNewBillingResponse {
-        canceled_subscription_id: stripe_subscription_id.to_string(),
+        canceled_subscription_id: canceled_subscription_id
+            .map(|subscription_id| subscription_id.to_string()),
     }))
 }