In the case of a non-existing property, the value would be undefined rather than null.

And while == and != exist in JavaScript, most linters will throw an error and require a === and !== instead as they should be avoided.

null == undefined // true
null === undefined // false

Besides, null is a perfectly valid value for a property, just as 0. Working with API Platform, I couldn't tell the number of times I used this kind of statement:

if (property || property === null) {
  // do some stuff
}

Probably just as much as

if (property || property === 0) {
  // do some stuff
}