Why can't an airplane stall at zero G?
At 0 G the plane does not need to generate any lift, therefore there is no critical angle past which the airfoil cannot generate the lift required.

In addition, a sustained 0 G maneuver is a parabolic arc, which means that in theory your flight path should keep your angle of attack at zero the entire time (in a theoretical, symmetrical airfoiled aicraft). You can't stall because your AoA is constantly 0 degrees and cannot exceed the critical angle.