Fixed unnecessary zero-length array allocations

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

OpenRA.Game/StreamExts.cs

@@ -229,7 +229,7 @@ namespace OpenRA
 			if (!string.IsNullOrEmpty(text))
 				bytes = encoding.GetBytes(text);
 			else
-				bytes = new byte[0];
+				bytes = Array.Empty<byte>();
 
 			s.Write(bytes.Length);
 			s.WriteArray(bytes);