Fix search being broken in channel listing "channel"

osu.Game/Online/Chat/Channel.cs

@@ -12,6 +12,7 @@ using osu.Framework.Bindables;
 using osu.Framework.Lists;
 using osu.Game.Online.API.Requests.Responses;
 using osu.Game.Overlays.Chat;
+using osu.Game.Overlays.Chat.Listing;
 
 namespace osu.Game.Online.Chat
 {
@@ -86,8 +87,11 @@ namespace osu.Game.Online.Chat
         [JsonProperty(@"last_read_id")]
         public long? LastReadId;
 
+        /// <remarks>
+        /// Purposefully nullable for the sake of <see cref="ChannelListing.ChannelListingChannel"/>.
+        /// </remarks>
         [JsonProperty(@"message_length_limit")]
-        public int MessageLengthLimit;
+        public int? MessageLengthLimit;
 
         /// <summary>
         /// Signals if the current user joined this channel or not. Defaults to false.