Add property requirements to safe traits

OpenRA.Mods.Common/Traits/Sound/AmbientSound.cs

@@ -15,6 +15,7 @@ namespace OpenRA.Mods.Common.Traits
 	[Desc("Plays a looping audio file at the actor position. Attach this to the `World` actor to cover the whole map.")]
 	class AmbientSoundInfo : ITraitInfo
 	{
+		[FieldLoader.Require]
 		public readonly string SoundFile = null;
 
 		public object Create(ActorInitializer init) { return new AmbientSound(init.Self, this); }