The bug

When creating a world and leaving the world name blank, the "Create New World" button is disabled. However, when returning from the "Customize World" screen, the button is enabled again, allowing the player to create a world with an empty name.

This does not cause further issues though; the world is just saved as "World".

To reproduce

1. Start Minecraft
2. Click on "Singleplayer"
3. Click on "Create New World"
4. Clear the "World Name" field
   → The "Create New World" button is disabled
5. Click on "More World Options..."
6. Change "World Type" to something customizable, such as "Superflat" or "Buffet"
7. Click on "Customize..."
8. Click on "Cancel"
   → The "Create New World" button is enabled
9. Click on "Done"
   → The "Create New World" button is enabled and the "World Name" field is left blank

Video

https://youtu.be/Ma4aUwNi7C8