Fix #347

This comit makes `Content` optional for webhook execution. This comit also adds null checks to content when creating the api args to properly specify the optional struct to the model. This is done so the message entity doesn't try to parse a null string.

src/Discord.Net.Rest/API/Rest/CreateWebhookMessageParams.cs

@@ -13,7 +13,7 @@ namespace Discord.API.Rest
         private static JsonSerializer _serializer = new JsonSerializer { ContractResolver = new DiscordContractResolver() };
 
         [JsonProperty("content")]
-        public string Content { get; set;  }
+        public Optional<string> Content { get; set;  }
 
         [JsonProperty("nonce")]
         public Optional<string> Nonce { get; set; }

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -786,8 +786,9 @@ namespace Discord.API
             if (!args.Embeds.IsSpecified || args.Embeds.Value == null || args.Embeds.Value.Length == 0)
                 Preconditions.NotNullOrEmpty(args.Content, nameof(args.Content));
 
-            if (args.Content?.Length > DiscordConfig.MaxMessageSize)
+            if (args.Content.IsSpecified && args.Content.Value?.Length > DiscordConfig.MaxMessageSize)
                 throw new ArgumentException(message: $"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", paramName: nameof(args.Content));
+
             options = RequestOptions.CreateOrClone(options);
 
             var ids = new BucketIds(webhookId: webhookId);
@@ -1336,7 +1337,7 @@ namespace Discord.API
             if ((!args.Embeds.IsSpecified || args.Embeds.Value == null || args.Embeds.Value.Length == 0) && !args.File.IsSpecified)
                 Preconditions.NotNullOrEmpty(args.Content, nameof(args.Content));
 
-            if (args.Content?.Length > DiscordConfig.MaxMessageSize)
+            if(args.Content.IsSpecified && args.Content.Value?.Length > DiscordConfig.MaxMessageSize)
                 throw new ArgumentException(message: $"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", paramName: nameof(args.Content));
 
             options = RequestOptions.CreateOrClone(options);

src/Discord.Net.Rest/Entities/Interactions/CommandBase/RestCommandBase.cs

@@ -115,7 +115,7 @@ namespace Discord.Rest
                 Type = InteractionResponseType.ChannelMessageWithSource,
                 Data = new API.InteractionCallbackData
                 {
-                    Content = text,
+                    Content = text ?? Optional<string>.Unspecified,
                     AllowedMentions = allowedMentions?.ToModel() ?? Optional<API.AllowedMentions>.Unspecified,
                     Embeds = embeds.Select(x => x.ToModel()).ToArray(),
                     TTS = isTTS,

src/Discord.Net.WebSocket/Entities/Interaction/SocketBaseCommand/SocketCommandBase.cs

@@ -114,7 +114,7 @@ namespace Discord.WebSocket
                 Type = InteractionResponseType.ChannelMessageWithSource,
                 Data = new API.InteractionCallbackData
                 {
-                    Content = text,
+                    Content = text ?? Optional<string>.Unspecified,
                     AllowedMentions = allowedMentions?.ToModel() ?? Optional<API.AllowedMentions>.Unspecified,
                     Embeds = embeds.Select(x => x.ToModel()).ToArray(),
                     TTS = isTTS,
@@ -168,7 +168,7 @@ namespace Discord.WebSocket
 
             var args = new API.Rest.CreateWebhookMessageParams
             {
-                Content = text,
+                Content = text ?? Optional<string>.Unspecified,
                 AllowedMentions = allowedMentions?.ToModel() ?? Optional<API.AllowedMentions>.Unspecified,
                 IsTTS = isTTS,
                 Embeds = embeds.Select(x => x.ToModel()).ToArray(),