Fix NRE in RestGuild

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -195,20 +195,20 @@ namespace Discord.Rest
 
             if (model.Stickers != null)
             {
-                var stickers = new ImmutableArray<CustomSticker>();
+                var stickers = ImmutableArray.CreateBuilder<CustomSticker>();
                 for (int i = 0; i < model.Stickers.Length; i++)
                 {
                     var sticker = model.Stickers[i];
-                    
+
                     var entity = CustomSticker.Create(Discord, sticker, this, sticker.User.IsSpecified ? sticker.User.Value.Id : null);
 
                     stickers.Add(entity);
                 }
 
-                _stickers = stickers;
+                _stickers = stickers.ToImmutable();
             }
             else
-                _stickers = new ImmutableArray<CustomSticker>();
+                _stickers = ImmutableArray.Create<CustomSticker>();
 
             Available = true;
         }