Support nullptr_t

[Chlumsky]

I have realized that when skipEmptyFields is true, there is no way to output null into the JSON. A possible solution for this very niche use case would be to add a NullType class that would be used for std::nullptr_t and would always serialize as null and when parsed, it would just throw TYPE_MISMATCH if the value is anything else. I would like to add this mainly because it's a pretty elegant use of the available nullptr_t type.