Can you cast Hangarback Walker for X=0 and have it survive with Hardened Scales?
No, and for a very straightforward reason: Hardened Scales requires that at least one counter be placed on the creature before it adds any additional counters:



The key text here is 'If one or more'.  You're trying to put zero counters on the Hangarback Walker, and so Hardened Scales never processes.