Fix NPC not showing for players without skin when using @mirror

[Bruhdows]

📋 Description

When an NPC uses the @mirror skin setting and is viewed by an offline/cracked player without a skin, the NPC wouldn't render. This fixes it by falling back to the NPC's own skin data if the viewer doesn't have one.

✅ Checklist

• My code follows the project's coding style and guidelines
• I have tested my changes locally and they work as expected
• I have added necessary documentation (no documentation changes needed - bug fix)
• I have rebased/merged with the latest main branch

🔍 Changes

• Added fallback to NPC's own skin data when viewer's profile lacks textures
• Ensures NPCs render correctly regardless of viewer's skin availability

🧪 How to Test

1. Create an NPC with @mirror skin setting
2. View it as an offline/cracked player (or test player without skin)
3. Verify the NPC renders with its own skin instead of disappearing