Change Explodes.Weapon default to null

FieldLoader.Require makes the default pointless, so it makes more sense to change it to null.

OpenRA.Mods.Common/Traits/Explodes.cs

@@ -23,7 +23,7 @@ namespace OpenRA.Mods.Common.Traits
 	public class ExplodesInfo : ConditionalTraitInfo, Requires<HealthInfo>
 	{
 		[WeaponReference, FieldLoader.Require, Desc("Default weapon to use for explosion if ammo/payload is loaded.")]
-		public readonly string Weapon = "UnitExplode";
+		public readonly string Weapon = null;
 
 		[WeaponReference, Desc("Fallback weapon to use for explosion if empty (no ammo/payload).")]
 		public readonly string EmptyWeapon = "UnitExplode";