C#: Consider the extraction of empty binlog files acceptable

[tamasvajk]

This PR changes how empty binlog files are treated. Previously, the extractor returned a failed extraction error code in such a case:

[ERROR] Spawned process exited abnormally (code 2; tried to run: [.../extractor-csharp/csharp/tools/autobuild.sh])
A fatal error occurred: Exit status 2 from command: [...extractor-csharp/csharp/tools/autobuild.sh]

Now the behavior matches what we experience in traced mode, when no source code is found:

Finalizing database at .../DB1.
CodeQL did not detect any code written in languages supported by this CodeQL distribution (C#, XML, Java Properties Files or Java/Kotlin). Confirm that there is some source code for one of these languages in the project. For more information, review our troubleshooting guide at https://gh.io/troubleshooting-code-scanning/no-source-code-seen-during-build.

The exit code is:

❯ echo $?         
32

This change is in response to #17981 (comment).