fix: Move content check for ModifyMessage (#1503)

good catch, thanks!

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -602,13 +602,8 @@ namespace Discord.API
             Preconditions.NotEqual(channelId, 0, nameof(channelId));
             Preconditions.NotEqual(messageId, 0, nameof(messageId));
             Preconditions.NotNull(args, nameof(args));
-            if (args.Content.IsSpecified)
-            {
-                if (!args.Embed.IsSpecified)
-                    Preconditions.NotNullOrEmpty(args.Content, nameof(args.Content));
-                if (args.Content.Value?.Length > DiscordConfig.MaxMessageSize)
-                    throw new ArgumentOutOfRangeException($"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", nameof(args.Content));
-            }
+            if (args.Content.IsSpecified && args.Content.Value?.Length > DiscordConfig.MaxMessageSize)
+                throw new ArgumentOutOfRangeException($"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", nameof(args.Content));
             options = RequestOptions.CreateOrClone(options);
 
             var ids = new BucketIds(channelId: channelId);

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

@@ -32,6 +32,11 @@ namespace Discord.Rest
             var args = new MessageProperties();
             func(args);
 
+            bool hasText = args.Content.IsSpecified ? !string.IsNullOrEmpty(args.Content.Value) : !string.IsNullOrEmpty(msg.Content);
+            bool hasEmbed = args.Embed.IsSpecified ? args.Embed.Value != null : msg.Embeds.Any();
+            if (!hasText && !hasEmbed)
+                Preconditions.NotNullOrEmpty(args.Content.IsSpecified ? args.Content.Value : string.Empty, nameof(args.Content));
+
             var apiArgs = new API.Rest.ModifyMessageParams
             {
                 Content = args.Content,