hotfix for ConvertMessage

+ added nullable support

src/PipelineMessagePublisher.cs

@@ -29,7 +29,7 @@ public class PipelineMessagePublisher<TMessage> :
     /// </summary>
     public IDisposable Subscribe<TDest>(TranslatorBase<TMessage, TDest> translator)
     {
-        return Subscribe(translator);
+        return Subscribe((ISubscriber<TMessage>)translator);
     }
 
     /// <summary>

src/Translating/TranslatorBase.cs

@@ -8,9 +8,28 @@ public abstract class TranslatorBase<TSource, TDest>(ISubscriber<TDest> destSubs
     public void OnCompleted() => destSubscriber.OnCompleted();
 
     /// <inheritdoc/>
-    public void OnNext(TSource message, NextDelegate next) => destSubscriber.OnNext(ConvertMessage(message), next);
+    public void OnNext(TSource message, NextDelegate next) 
+    {
+        var dest = ConvertMessage(message);
+        if (dest != null)
+        {
+            destSubscriber.OnNext(dest, next);
+        }
+        else
+        {
+            next();
+        }
+    }
 
-    protected abstract TDest ConvertMessage(TSource message);
+    /// <summary>
+    /// Converts a message from one type to another.
+    /// </summary>
+    /// <param name="message"> Source message. </param>
+    /// <returns> Destination message or null if unconvertable. </returns>
+    /// <remarks>
+    /// - The NextDelegate is triggered by target subsriber, or by this class if unconvertable.
+    /// </remarks>
+    protected abstract TDest? ConvertMessage(TSource message);
 
     public static TranslatorBase<TSource, TDest> Create(ISubscriber<TDest> destSubscriber, Func<TSource, TDest> converter) 
         => new SimpleTranslator<TSource, TDest>(destSubscriber, converter);

src/package.nuspec

@@ -3,7 +3,7 @@
   <metadata>
     <id>ProSol.Messaging</id>
     <title>ProSol.Messaging</title>
-    <version>4.0.0-rc.1</version>
+    <version>4.0.0-rc.2</version>
     <authors>Alex Kozachenko</authors>
     <owners>Alex Kozachenko</owners>
     <projectUrl> https://git.disroot.org/alexenko/ProSol.Messaging </projectUrl>