Abort video download process if no videos are found

[Gudauu]

This pull request introduces a check for an empty video_urls list before the video downloading process begins. If no videos are found, the process is aborted with an appropriate error message, following the existing error handling pattern.

This prevents unnecessary attempts to download when there are no videos and provides clear feedback to the user.

Changes:

• Added a check for an empty video_urls list in /Backend/main.py before downloading.
• If the list is empty, the process is aborted with a JSON response indicating the error.