Fix DivideByZeroException when compressing empty files with BZip2

[Copilot]

Writing empty files or folders to a Zip archive with BZip2 compression throws DivideByZeroException in MainSort() due to (i % (last + 1)) evaluating to (i % 0) when last = -1.

var options = new WriterOptions(CompressionType.BZip2);
using var writer = WriterFactory.Open(stream, ArchiveType.Zip, options);
writer.Write("empty-folder/", Stream.Null);  // throws DivideByZeroException

Changes

• CBZip2OutputStream.cs: Early return in EndBlock() when last < 0 to skip block processing for empty input
• ZipWriterTests.cs: Added tests for empty file and empty folder scenarios with BZip2 compression

Original prompt

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This section details on the original issue you should resolve

<issue_title>DivideByZeroException throw when using Zip with BZip2</issue_title>
<issue_description>Should fixed this issue since we need to put empty folder or empty files into Zip.
Which is very common scenario. Because you would never know which kinds of data that user wants to compress.

On the other hand, Zip allows empty file or empty folder. We could not filter away those zero-byte files or empty folders which would be very confusing.

WriterOptions Options = new WriterOptions(CompressionType.BZip2)
{
    ArchiveEncoding = new ArchiveEncoding
    {
        Default = new UTF8Encoding(false)
    }
};

using (IWriter Writer = WriterFactory.Open(<OutputStream>, ArchiveType.Zip, Options))
{
     Writer.Write("test-empty-folder/", Stream.Null); // Cause DivideByZeroException
     Writer.Write("test-folder/zero-byte-file.txt", Stream.Null); // Cause DivideByZeroException
}

last is -1 here

sharpcompress/src/SharpCompress/Compressors/BZip2/CBZip2OutputStream.cs

Line 1417 in 9433e06

block[last + i + 2] = block[(i % (last + 1)) + 1];

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes DivideByZeroException throw when using Zip with BZip2 #1039

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Pull request overview

This PR fixes a DivideByZeroException that occurs when writing empty files or folders to a Zip archive using BZip2 compression. The exception was caused by the modulo operation (i % (last + 1)) in MainSort() evaluating to (i % 0) when last = -1 (indicating no data was written to the block).

• Added an early return in CBZip2OutputStream.EndBlock() to skip block processing when no data has been written (last < 0)
• Added test coverage for both empty file and empty folder scenarios with BZip2 compression

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/SharpCompress/Compressors/BZip2/CBZip2OutputStream.cs	Adds early return in EndBlock() to prevent divide-by-zero when compressing empty input
tests/SharpCompress.Test/Zip/ZipWriterTests.cs	Adds regression tests for empty file and folder scenarios with BZip2 compression

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