Fix CA1825 warnings on empty array initialisation.

OpenRA.Mods.D2k/Traits/Buildings/D2kActorPreviewPlaceBuildingPreview.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using OpenRA.Graphics;
@@ -27,7 +28,7 @@ namespace OpenRA.Mods.D2k.Traits
 		public readonly HashSet<string> UnsafeTerrainTypes = new HashSet<string> { "Rock" };
 
 		[Desc("Only check for 'unsafe' footprint tiles when you have these prerequisites.")]
-		public readonly string[] RequiresPrerequisites = { };
+		public readonly string[] RequiresPrerequisites = Array.Empty<string>();
 
 		[Desc("Sprite image to use for the overlay.")]
 		public readonly string Image = "overlay";