Allow null value to reset IGuildUser nickname (#923)

* Added workaround for UserHelper#ModifyAsync that accepts null values as a way to reset user nicknames

* Update comments

* Update comment to use see tag

src/Discord.Net.Core/Entities/Users/GuildUserProperties.cs

@@ -34,7 +34,7 @@ public class GuildUserProperties
         /// Should the user have a nickname set? 
         /// </summary>
         /// <remarks>
-        /// To clear the user's nickname, this value can be set to null.
+        /// To clear the user's nickname, this value can be set to <see langword="null" /> or <see cref="string.Empty" />.
         /// </remarks>
         public Optional<string> Nickname { get; set; }
         /// <summary>

src/Discord.Net.Rest/Entities/Users/UserHelper.cs

@@ -48,6 +48,14 @@ internal static class UserHelper
             else if (args.RoleIds.IsSpecified)
                 apiArgs.RoleIds = args.RoleIds.Value.ToArray();
 
+            /*
+             * Ensure that the nick passed in the params of the request is not null.
+             * string.Empty ("") is the only way to reset the user nick in the API,
+             * a value of null does not. This is a workaround.
+             */
+            if (apiArgs.Nickname.IsSpecified && apiArgs.Nickname.Value == null)
+                apiArgs.Nickname = new Optional<string>(string.Empty);
+
             await client.ApiClient.ModifyGuildMemberAsync(user.GuildId, user.Id, apiArgs, options).ConfigureAwait(false);
             return args;
         }