Fixed unnecessary zero-length array allocations

Changed all currently present zero-length array allocations in the codebase to use `Array.Empty` instead.

OpenRA.Mods.Common/SpriteLoaders/TgaLoader.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -78,7 +79,7 @@ namespace OpenRA.Mods.Common.SpriteLoaders
 
 			public TgaFrame()
 			{
-				Data = new byte[0];
+				Data = Array.Empty<byte>();
 			}
 
 			public TgaFrame(Stream stream)