add TrimOrEmpty to avoid NREs

OpenRA.Game/Exts.cs

@@ -30,6 +30,11 @@ namespace OpenRA
 			return string.Format(fmt, args);
 		}
 
+		public static string TrimOrEmpty(this string input)
+		{
+			return input != null ? input.Trim() : string.Empty;
+		}
+
 		public static T WithDefault<T>(T def, Func<T> f)
 		{
 			try { return f(); }