null vs missing key in REST API Response [closed]

Question

Say in my application, some users give us their last name, and others do not. In a REST API response, which body is preferred:

With a "null" value:

{"firstName": "Bob",
 "lastName": null}

Or just a missing key:

{"firstName": "Bob"}

Answer 1

Consider removing empty or null values.

If a property is optional or has an empty or null value, consider dropping the property from the JSON, unless there's a strong semantic reason for its existence.

{
  "volume": 10,

  // Even though the "balance" property's value is zero, it should be left in,
  // since "0" signifies "even balance" (the value could be "-1" for left
  // balance and "+1" for right balance.
  "balance": 0,

  // The "currentlyPlaying" property can be left out since it is null.
  // "currentlyPlaying": null
}

Further Reading
Google Style Guide - Empty or Null Property Values
Should null values be included in JSON responses from a REST API?