[VariantUtils] Use read_json to read negative numbers will result in positive numbers.

[JX-Master]

test code:

Variant var1((i64)-3);
String s = VariantUtils::write_json(var1);
R<Variant> var2 = VariantUtils::read_json(s.c_str());
luassert_always(succeeded(var2));
luassert_always(var1 == var2.get());