Never generate $0.00 invoices

Summary: Ref T7769. It might possibly make sense to generate these some day, but for now they aren't payable and we have no use for them.

Test Plan:
  - Generated $0.00 invoices, saw code decline to invoice.
  - Generated a $7.99 invoice, got an invoice.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7769

Differential Revision: https://secure.phabricator.com/D12878

src/applications/phortune/worker/PhortuneSubscriptionWorker.php

@@ -15,6 +15,13 @@ final class PhortuneSubscriptionWorker extends PhabricatorWorker {
       return;
     }
 
+    $currency = $subscription->getCostForBillingPeriodAsCurrency(
+      $last_epoch,
+      $next_epoch);
+    if (!$currency->isPositive()) {
+      return;
+    }
+
     $account = $subscription->getAccount();
     $merchant = $subscription->getMerchant();
 
@@ -48,10 +55,6 @@ final class PhortuneSubscriptionWorker extends PhabricatorWorker {
 
     $purchase = $cart->newPurchase($actor, $product);
 
-    $currency = $subscription->getCostForBillingPeriodAsCurrency(
-      $last_epoch,
-      $next_epoch);
-
     $purchase
       ->setBasePriceAsCurrency($currency)
       ->setMetadataValue('subscriptionPHID', $subscription->getPHID())