Actually you can't. If you choose not to set a pin, Signal just chooses one for you and uses that to upload all your data, only you won't be able to access it. There is no way to prevent your data from being sent to the cloud. For more info see here: https://old.reddit.com/r/signal/comments/htmzrr/psa_disablin... and https://community.signalusers.org/t/what-contact-info-does-t...

The fact that Signal users are still unaware of where their data is going and when should tell you all you need to know about how trustworthy the service is. Not being 100% clear about the risks people take when using software which is promoted for use by people whose freedom and/or lives depend on it being secure is a very bad look for Signal.

As for message backups they are at least opt-in (for now anyway) and you can learn more about them here: https://signal.org/blog/introducing-secure-backups/