Adds fallback to app_name_from_production_database_name

[emars]

app_name_from_production_database_name can sometimes return nil if your config does not match the expected format.

This pull request adds a default to this method so the generator does not fail when calling app_name.upcase.

[dhh]

I'd actually rather follow the lead from solid_queue and yank this munging all together. Rails 8 will do this configuration directly in config/database.yml out of the box, and I think it's too likely that we won't match things right otherwise. So should update the README to just be explicit about what you need to add to config/database.yml.