I regularly need two types of "null" and both JSON and XML among others provide it:

1. Unknown value, the user whatever could not provide a value so it was left blank, or it had a value and was purposely removed (as in an update), but it was set by the other side specifically to null.

2. The value was not not provided, that is it was not set by the other side, missing property or element (undefined). This usually means do not modify the value if doing an update, distinct from setting it too null.