Our convention is currently to use null for nully values.  I think the main win on this is that JSON will transit null values which can be useful for debugging, whereas undefined values get dropped, which can be confusing.  Also, it's nice to be able to know that when you see 'undefined', there is probably a typo somewhere that accessed a non-existent property.  (Which is in turn nice because it helps you make sure you are creating consistent shapes by initializing the object consistently.)