Fix val.py 'no labels found bug'

[glenn-jocher]

Resolves #8791

Bug first introduced in #8782

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Refinement of validation script with code simplifications for readability.

📊 Key Changes

• Changed the way names dictionary is created for better clarity.
• Simplified the string formatting for saving text files by using f-string.
• Removed unnecessary conditional branch to always define nt (number of targets per class) outside of condition.

🎯 Purpose & Impact

• Improving code readability and simplicity for maintenance and future contributions.
• Potential reduction in errors due to simpler code structure.
• Users should experience no changes in functionality, ensuring consistent performance with minor behind-the-scenes improvements. 🛠️