Fixes bug where if you're not a moderator it errors

Author: chuggafan

RedditSharp/Things/AuthenticatedUser.cs

@@ -125,7 +125,7 @@ internal override JToken GetJsonData(JToken json) {
         /// enrolled in new modmail.
         /// </summary>
         [JsonProperty("new_modmail_exists")]
-        public bool NewModmailExists { get; private set; }
+        public bool? NewModmailExists { get; private set; }
 
         /// <summary>
         /// Returns trus if this user is in the reddit beta program.