Fixes a bug causing infinite run when using conditional with empty body

[gadial]

Summary

This PR fixes a bug where using a conditional with an empty body causes an infinite loop when running the simulator.

Fixes #2230

Details and comments

When using a conditional with an empty body, e.g.

with main_circ.if_test((creg_0[0],0)) as else_1:
    pass
with else_1:
    pass

The simulator gets stuck in an infinite loop.

It seems the underlying cause of the problem is that when a conditional command has an empty body, it has empty qargs list as well. This means that the jump and mark commands generated by Aer will have empty qubits list and hence after a transpiler pass they may be relocated nondeterministically to any part of the circuit (they are usually placed between two barriers, but without qubits the DAG-to-circuit algorithm will not consider the barriers part of their topological order).

The attempted fix is adding

if len(qargs) == 0:
            qargs = parent.qubits

To ensure the commands remain between the barriers.