My partner works in HR. If you have given them a salary figure previously and they do this check and find you have lied then they can withdraw their offer.
This is why this check is in with the background check and why you should never give salary history to a potential employer.
Null value FTW.
They will still contact your previous employer but all that they will confirm is whether you worked there, no one will give out your previous salary. And no, I wouldn't give any supporting docs as per patio.