it's a belief system, based on a lack of belief rather than the alternative, but it is a belief system non the less.
No.
Then not collecting baseball cards is a 'hobby', and not smoking is a 'habit'
If I have a 'belief system', it would be IMO 'the best way to find out what is true and what is not'. Physical evidence, human reason and logic.
Since this is my 'belief system', my stance on certain claims like, gods or fire breathing dragons is a 'lack of belief'. So my stance on each claim would not be a 'belief system' unto itself.
Hope I cleared it up.