Fix NPCs stealing player default name

[ShnitzelX2]

Summary

Bugfixes "NPCs stealing player default name"

Purpose of change

Indeed a bug caused by #76935

Closes #77235

Character::pick_name() is used in Character::randomize(), so NPCs use it now. It can set name to the option-defined default name (DEF_CHAR_NAME), which is not intended for NPCs.

Describe the solution

Add is_avatar() check, one-liner.

Describe alternatives you've considered

Testing

With default name as "Default Name!":

Starting game with "Play Now!":

• '@' menu
  
• some NPCs generated
  

Starting game with "Random Character":

And the same with empty default name:

• play now
  

• random character
  
  

Additional context