fix: Check error 404 and return null for GetBanAsync (#1614)

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -874,9 +874,13 @@ namespace Discord.API
             Preconditions.NotEqual(guildId, 0, nameof(guildId));
             options = RequestOptions.CreateOrClone(options);
 
+            try
+            {
                 var ids = new BucketIds(guildId: guildId);
                 return await SendAsync<Ban>("GET", () => $"guilds/{guildId}/bans/{userId}", ids, options: options).ConfigureAwait(false);
             }
+            catch (HttpException ex) when (ex.HttpCode == HttpStatusCode.NotFound) { return null; }
+        }
         /// <exception cref="ArgumentException">
         /// <paramref name="guildId"/> and <paramref name="userId"/> must not be equal to zero.
         /// -and-

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

@@ -132,7 +132,7 @@ namespace Discord.Rest
         public static async Task<RestBan> GetBanAsync(IGuild guild, BaseDiscordClient client, ulong userId, RequestOptions options)
         {
             var model = await client.ApiClient.GetGuildBanAsync(guild.Id, userId, options).ConfigureAwait(false);
-            return RestBan.Create(client, model);
+            return model == null ? null : RestBan.Create(client, model);
         }
 
         public static async Task AddBanAsync(IGuild guild, BaseDiscordClient client,