message: checkered_flag: trying to fix C2440 error
  Error in AppVeyor:
  unit-conversions.cpp(441): error C2440: 'initializing': cannot convert from 'nlohmann::json' to 'std::nullptr_t'
  (https://ci.appveyor.com/project/nlohmann/json/build/2838/job/wo7im01sq6tvhe9m)
diff: @@ -438,7 +438,7 @@ TEST_CASE("value conversion")
  std::nullptr_t n;
  json j(n);
  - std::nullptr_t n2 = j;
  + std::nullptr_t n2 = (std::nullptr_t)j;
  CHECK(n2 == n);
  }
  