Don't build minidex if no sprites folder
Don't build minidex if no sprites folder

This prevents PS from disabling sprite animations if someone runs
`./build full` without having sprites.

Zarel/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client