Fix empty and directory-only 7z archive finalization

src/SharpCompress/Writers/SevenZip/SevenZipWriter.Async.cs

@@ -43,6 +43,8 @@ public partial class SevenZipWriter
 
     private async ValueTask FinalizeArchiveAsync()
     {
+        // Reserve the signature even when no file data was written.
+        await EnsurePlaceholderWrittenAsync(CancellationToken.None).ConfigureAwait(false);
         var output = OutputStream.NotNull();
 
         // Current position = end of packed data streams

src/SharpCompress/Writers/SevenZip/SevenZipWriter.cs

@@ -193,6 +193,8 @@ public partial class SevenZipWriter : AbstractWriter
 
     private void FinalizeArchive()
     {
+        // Directory-only and empty archives never pass through Write().
+        EnsurePlaceholderWritten();
         var output = OutputStream.NotNull();
 
         // Current position = end of packed data streams