[in_app_purchase] Prepare for NULLABLE_ARGUMENT_TO_NON_NULL_TYPE (flutter#3275)

[in_app_purchase] Prepare for `NULLABLE_ARGUMENT_TO_NON_NULL_TYPE`

Author: asashour

packages/in_app_purchase/in_app_purchase_storekit/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.6+3
+
+* Adds a null check, to prevent a new diagnostic.
+
 ## 0.3.6+2
 
 * Clarifies explanation of endorsement in README.
@@ -13,7 +17,7 @@
 
 ## 0.3.5+2
 
-* Fix a crash when `appStoreReceiptURL` is nil.
+* Fixes a crash when `appStoreReceiptURL` is nil.
 
 ## 0.3.5+1
 

packages/in_app_purchase/in_app_purchase_storekit/pubspec.yaml

@@ -2,7 +2,7 @@ name: in_app_purchase_storekit
 description: An implementation for the iOS and macOS platforms of the Flutter `in_app_purchase` plugin. This uses the StoreKit Framework.
 repository: https://github.com/flutter/packages/tree/main/packages/in_app_purchase/in_app_purchase_storekit
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+in_app_purchase%22
-version: 0.3.6+2
+version: 0.3.6+3
 
 environment:
   sdk: ">=2.18.0 <4.0.0"

packages/in_app_purchase/in_app_purchase_storekit/test/fakes/fake_storekit_platform.dart

@@ -163,7 +163,9 @@ class FakeStoreKitPlatform {
         return Future<void>.sync(() {});
       case '-[InAppPurchasePlugin retrieveReceiptData:result:]':
         if (receiptData != null) {
-          return Future<String>.value(receiptData);
+          // TODO(asashour): Remove the `ignore` when Dart 3 reaches stable.
+          // ignore: unnecessary_null_checks
+          return Future<String>.value(receiptData!);
         } else {
           throw PlatformException(code: 'no_receipt_data');
         }