[core] Only ensure playlist thumbnail dir if writing thumbs

[bashonly]

Only create directories for playlist thumbs when they have been requested for download

Bugfix for 2acd1d5

Closes #8372

Template

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Copilot Summary

🤖 Generated by Copilot at 8c07484

Summary

🐛🚫🗂️

Prevent creating empty directories for thumbnails when none are available or requested. Add a check for empty thumbnails list in yt_dlp/YoutubeDL.py.

No thumbnails, no dir
_ensure_dir_exists waits
Empty folders, gone

Walkthrough

• Prevent creating empty directories for thumbnails when none are available or requested (link)

[Riteo]

oops, sorry!

[bashonly]

@Riteo all good, I missed it in original PR's code review too