Not sure what you mean here by "most JavaScript parser rejects null" - did you mean "JSON parsers"? And why would they reject null, which is a valid JSON value?

It's more that when building an API that adheres to a specification, whether formal or informal, if the field is supposed to be a JSON array then it should be a JSON array. Not _sometimes_ a JSON array and _sometimes_ null, but always an array. That way clients consuming the JSON output can write code consuming that array without needing to be overly defensive