It can happen to either type. The rebar is within the gunite and the whole shell acts like a boat if there is significant ground water around and the whole thing floats. It is fairly rare though.

'Weep' holes can be installed (drilled through pool floor) to allow the water a path through to equalize the pressure on the shell whilst it's empty.

My (gunite) pool is in the process of being built and has two such 'weep' holes through the gunite and will have until it is plastered at which point they'll be filled in.

I'm not sure I'd want to leave the pool empty for a prolonged period if it were me.