Merge pull request #854 from zgabi/patch-1

return Stream.Null when 7z entry has no stream

src/SharpCompress/Common/SevenZip/SevenZipFilePart.cs

@@ -40,7 +40,7 @@ internal class SevenZipFilePart : FilePart
     {
         if (!Header.HasStream)
         {
-            throw new InvalidFormatException("File does not have a stream.");
+            return Stream.Null;
         }
         var folderStream = _database.GetFolderStream(_stream, Folder!, _database.PasswordProvider);