Fix ModifyAsync (#2023)

* Fix issue with Argument cannot be blank when modifying flags

* Make uniform with existing code

src/Discord.Net.Rest/Entities/Messages/MessageHelper.cs

@@ -42,8 +42,10 @@ namespace Discord.Rest
             bool hasEmbeds = embed.IsSpecified && embed.Value != null || embeds.IsSpecified && embeds.Value?.Length > 0;
             bool hasComponents = args.Components.IsSpecified && args.Components.Value != null;
             bool hasAttachments = args.Attachments.IsSpecified;
+            bool hasFlags = args.Flags.IsSpecified;
 
-            if (!hasComponents && !hasText && !hasEmbeds && !hasAttachments)
+            // No content needed if modifying flags
+            if ((!hasComponents && !hasText && !hasEmbeds && !hasAttachments) && !hasFlags)
                 Preconditions.NotNullOrEmpty(args.Content.IsSpecified ? args.Content.Value : string.Empty, nameof(args.Content));
 
             if (args.AllowedMentions.IsSpecified)