review null or undefined se/deserialization expression

[qiaozha]

Currently, if a property is null or undefined type, we will generate the code as

    functions: !item["functions"]
      ? item["functions"]
      : functionDefinitionArraySerializer(item["functions"]),

which could be problematic if item["functions"] is an empty string or number 0 or boolean false, and it supposed to go into the serialization function logic. Like we get a 0 for timestamp type.

See comment here #2759 (comment)