Fix CA1825 warnings on empty array initialisation.

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

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Linq;
 using OpenRA.Traits;
 
@@ -24,7 +25,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		[FieldLoader.Require]
 		[Desc("List of required prerequisites.")]
-		public readonly string[] Prerequisites = { };
+		public readonly string[] Prerequisites = Array.Empty<string>();
 
 		public override object Create(ActorInitializer init) { return new GrantConditionOnPrerequisite(init.Self, this); }
 	}