JsonSerializer.NullValueHandling should be Include

[richardszalay]

Mandrill.net configures JSON.NET to use NullValueHandling.Ignore, but this causes problems with loops due to the crazy nature of how loop scoping works in Mandrill's handlebars implementation (the specifics of which are covered here and here, the latter of which is from 2016 and shows that the problem is unlikely to be solved by Mandrill any time soon).

I propose changing NullValueHandling to Include since it solves this scenario... for non-array properties.

(Mandrill's array property handling appears to be even worse, requiring a non-empty array to replace the previous iteration's value. The above change will obviously not help with that)