fix(7z): handle empty-stream entries in WriteToDirectory (fixes #1217)

src/SharpCompress/Archives/SevenZip/SevenZipArchive.cs

@@ -162,6 +162,15 @@ protected override EntryStream GetEntryStream()
                 return CreateEntryStream(Stream.Null);
             }
 
+            // 7z may contain valid "empty stream" file entries (HasStream == false).
+            // These entries represent zero-byte files and do not have folder mapping.
+            // Returning Stream.Null keeps extraction behavior correct and avoids
+            // null-folder dereference when requesting a folder stream.
+            if (!entry.FilePart.Header.HasStream)
+            {
+                return CreateEntryStream(Stream.Null);
+            }
+
             var folder = entry.FilePart.Folder;
 
             // Check if we're starting a new folder - dispose old folder stream if needed

tests/SharpCompress.Test/SevenZip/SevenZipArchiveTests.cs

@@ -132,6 +132,33 @@ public void SevenZipArchive_BZip2_Split_FirstFileRead() =>
     [Fact]
     public void SevenZipArchive_Copy_PathRead() => ArchiveFileRead("7Zip.Copy.7z");
 
+    [Fact]
+    public void SevenZipArchive_EmptyStream_WriteToDirectory_DoesNotThrow()
+    {
+        var archivePath = Path.Combine(TEST_ARCHIVES_PATH, "7Zip.EmptyStream.7z");
+        var outputPath = Path.Combine(
+            SCRATCH_FILES_PATH,
+            nameof(SevenZipArchive_EmptyStream_WriteToDirectory_DoesNotThrow));
+
+        if (Directory.Exists(outputPath))
+        {
+            Directory.Delete(outputPath, true);
+        }
+
+        Directory.CreateDirectory(outputPath);
+
+        using var archive = SevenZipArchive.OpenArchive(archivePath);
+        var progress = new Progress<ProgressReport>(_ => { });
+
+        var ex = Record.Exception(() => archive.WriteToDirectory(outputPath, progress));
+
+        Assert.Null(ex);
+
+        var files = Directory.GetFiles(outputPath, "*", SearchOption.AllDirectories);
+        Assert.NotEmpty(files);
+        Assert.Contains(files, p => new FileInfo(p).Length == 0);
+    }
+
     [Fact]
     public void SevenZipArchive_Copy_CompressionType()
     {