Handling empty purchases list on purchase update

[dogramacigokhan]

Issue Description

Fixing payment issue while the app is running in background throws NullReferenceException.

Underlying Reason

Fixing the payment issue triggers a new purchase update from native side with null purchases list when the application is running in background, but the C# side doesn't handle this situation well and throws a NullReferenceException. Actual underlying reason might be triggering the purchase update with null purchases list for this specific case, but considering the annotations in the native code and XML documentations in the C# code, having a null purchases list looks like a valid case and should be handled nonetheless.

Fix

Fix contains three main modifications:

Fix calling a method (size()) on a nullable list coming from native side

In GooglePlayBilling.aar, PurchasesUpdatedListener.onPurchasesUpdated() callback declares a nullable purchases list as follows when it's decompiled:

public interface PurchasesUpdatedListener {
    void onPurchasesUpdated(@NonNull BillingResult var1, @Nullable List<Purchase> var2);
}

but in JniUtils.ParseJavaPurchaseList(), there's no null check before calling the native size() method on it, hence it throws NullReferenceException.

Fix handling empty purchases list

JniUtils.ParseJavaPurchaseList() has following XML doc for the return value:

/// <returns>An IEnumerable of <cref="Purchase"/>. The IEnumerable could be empty.</returns>

but GooglePlayStoreImpl.ParsePurchaseResult() calls First() on the list, hence it throws InvalidOperationException when the list is empty.

Fix enumerating IEnumerable multiple times

GooglePlayStoreImpl.ParsePurchaseResult() enumerates IEnumerable purchases list multiple times (first with _inventory.UpdatePurchaseInventory() call and then with purchasesList.First() call). Added ToList() to evaluate it once and enumerate it multiple times safely.

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Related issue: https://issuetracker.google.com/issues/171860953

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[dogramacigokhan]

@googlebot I signed it!

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[dogramacigokhan]

Closing because of cla issues. For the correct PR, see: #64