pneumatic cannon fixes (#14705)

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Shared/PneumaticCannon/SharedPneumaticCannonSystem.cs

@@ -20,6 +20,13 @@ public abstract class SharedPneumaticCannonSystem : EntitySystem
 
     private void OnAttemptShoot(EntityUid uid, PneumaticCannonComponent component, ref AttemptShootEvent args)
     {
+        // if the cannon doesn't need gas then it will always predict firing
+        if (component.GasUsage == 0f)
+            return;
+
+        // pneumatic cannon usually doesn't shoot bullets
+        args.ThrowItems = component.ThrowItems;
+
         // we don't have atmos on shared, so just predict by the existence of a slot item
         // server will handle auto ejecting/not adding the slot item if it doesnt have enough gas,
         // so this won't mispredict