Fix #9358: TarScanner no longer ignores empty files

[SamuXarick]

Motivation / Problem

An AI requires a file with 0 bytes to be able to start, but the tar scanner skips empty files, causing the AI to fail with a "file not found" error.
See #9358

Description

The problem is solved by allowing tar scanner to add empty files.

Limitations

It was suggested to fix it this way, but there's some uncertainty about it. I'm not familiar with this code.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[LordAro]

Probably fine. Commit might want rewording on squash