Fix CA1825 warnings on empty array initialisation.

OpenRA.Mods.Common/Traits/Conditions/GrantConditionOnTileSet.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Linq;
 using OpenRA.Traits;
 
@@ -23,7 +24,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		[FieldLoader.Require]
 		[Desc("Tile set IDs to trigger the condition.")]
-		public readonly string[] TileSets = { };
+		public readonly string[] TileSets = Array.Empty<string>();
 
 		public override object Create(ActorInitializer init) { return new GrantConditionOnTileSet(this); }
 	}