Allow passing null to JsonArray.Add() (dotnet/corefx#25104)

Allow passing null to JsonArray.Add() (dotnet/corefx#25104)

* Allow passing null to JsonArray.Add()

null is a valid value in a JSON array, so there is no reason to forbid
adding null to the array.

JsonArray.Save() already handles null values in the array.

* Fix JsonArray test

Commit migrated from https://github.com/dotnet/corefx/commit/c0718f870f12493f8359a95443362ee208aeb8a0