Allow empty translation keys

OpenRA.Game/Translation.cs

@@ -88,8 +88,8 @@ namespace OpenRA
 
 		public bool TryGetString(string key, out string value, IDictionary<string, object> arguments = null)
 		{
-			if (string.IsNullOrEmpty(key))
-				throw new ArgumentException("A translation key must not be null or empty.", nameof(key));
+			if (key == null)
+				throw new ArgumentNullException(nameof(key));
 
 			try
 			{