Fixed nickname null check, hid internal method

src/Discord.Net/API/DiscordRestApiClient.cs

@@ -816,7 +816,7 @@ namespace Discord.API
         {
             return CreateMessageInternalAsync(0, channelId, args);
         }
-        public async Task<Message> CreateMessageInternalAsync(ulong guildId, ulong channelId, CreateMessageParams args, RequestOptions options = null)
+        private async Task<Message> CreateMessageInternalAsync(ulong guildId, ulong channelId, CreateMessageParams args, RequestOptions options = null)
         {
             Preconditions.NotEqual(channelId, 0, nameof(channelId));
             Preconditions.NotNull(args, nameof(args));
@@ -1016,7 +1016,7 @@ namespace Discord.API
         public async Task ModifyMyNickAsync(ulong guildId, ModifyCurrentUserNickParams args, RequestOptions options = null)
         {
             Preconditions.NotNull(args, nameof(args));
-            Preconditions.NotEmpty(args.Nickname, nameof(args.Nickname));
+            Preconditions.NotNull(args.Nickname, nameof(args.Nickname));
 
             await SendAsync("PATCH", $"guilds/{guildId}/members/@me/nick", args, options: options).ConfigureAwait(false);
         }