Allow content to be empty when sending a message

src/Discord.Net.Core/API/DiscordRestApiClient.cs

@@ -439,8 +439,7 @@ namespace Discord.API
         {
             Preconditions.NotEqual(channelId, 0, nameof(channelId));
             Preconditions.NotNull(args, nameof(args));
-            Preconditions.NotNullOrEmpty(args.Content, nameof(args.Content));
-            if (args.Content.Length > DiscordConfig.MaxMessageSize)
+            if (args.Content.GetValueOrDefault()?.Length > DiscordConfig.MaxMessageSize)
                 throw new ArgumentException($"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", nameof(args.Content));
             options = RequestOptions.CreateOrClone(options);
 

src/Discord.Net.Core/API/Rest/CreateMessageParams.cs

@@ -8,7 +8,7 @@ namespace Discord.API.Rest
     public class CreateMessageParams
     {
         [JsonProperty("content")]
-        public string Content { get; }
+        public Optional<string> Content { get; }
 
         [JsonProperty("nonce")]
         public Optional<string> Nonce { get; set; }
@@ -19,7 +19,10 @@ namespace Discord.API.Rest
 
         public CreateMessageParams(string content)
         {
-            Content = content;
+            if (string.IsNullOrEmpty(content))
+                Content = null;
+            else
+                Content = content;
         }
     }
 }