Ignore null elements when autopaginating

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Okay, so the bug here is a bit convoluted:

• list elements in the data attribute of a list object are decoded using StripeObjectConverter:

  stripe-dotnet/src/Stripe.net/Entities/StripeList.cs

  Lines 20 to 21 in c879198

  	[JsonProperty("data", ItemConverterType = typeof(StripeObjectConverter))]
  	public List<T> Data { get; set; }

• when encountering an object it doesn't know how to decode, StripeObjectConverter returns null:

  stripe-dotnet/src/Stripe.net/Infrastructure/JsonConverters/StripeObjectConverter.cs

  Lines 57 to 58 in c879198

  	// Couldn't find a concrete type to instantiate, return null.
  	return null;

  
  (This was a deliberate design decision, because new object types may be added to the API and older versions of the library should not crash upon encountering them.)
• the two above points mean that the Data list can contain null elements. It would be better if those null values were not added to the list at all, but unfortunately I don't think Newtonsoft.Json's ItemConverterType attribute lets us do that
• when autopaginating, we cast elements to IHasId to extract the ID
• of course, trying to access the ID on a null instance fails and throws a NullReferenceException

The fix is fairly simple: just skip over null elements in a page when autopaginating.

Also, for full context, I ran into this because stripe-mock returns a list element with object=alipay_account and the library does not have a model for this object type.

[brandur-stripe]

Thanks as usual for the detailed context!

[ob-stripe]

Released as 27.14.1.