imagine we send an empty server call (without data, due to some bug in ab tests, doesnt matter the reason). Imagine that the user intend to bounce. So after user lands on our website and user itself does nothing on it as he intent to leave the page (1st server call with page view), then this bug sends an empty server call, does AA count it not bounced visit or is it still bounced?
Not sure what you mean by an "empty" server call. E.g. when you send a s.t() without settings any props, eVars, events or other variables, the server call will still include basic information from the page itself, e.g. page URL, referrer, etc. So that's not an "empty" server call at all.
But to your question, basically as long as AA receives a subsequent server call, then that visit is not treated as a bounce.
Assuming that those "empty"server calls are sent as Custom Links, then if you need to calculate an actual Bounce Rate from pageviews, create a Calculated Metric with the formula: Single Page Visits / Entries. That will return a bounce rate based on pageview server calls only.