Check if weapon without self-reloading is out of ammo when deciding validity

This should prevent attacking helicopters (or other actors) staying at the range to target of a weapon that is out of ammo and needs manual reloading at a RearmBuilding or via upgrades.

OpenRA.Mods.Common/Traits/Armament.cs

@@ -259,6 +259,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		public bool IsReloading { get { return FireDelay > 0 || IsTraitDisabled; } }
 		public bool ShouldExplode(Actor self) { return !IsReloading; }
+		public bool OutOfAmmo { get { return ammoPool != null && !ammoPool.Info.SelfReloads && !ammoPool.HasAmmo(); } }
 
 		public WVec MuzzleOffset(Actor self, Barrel b)
 		{