Pretty sure it shouldn't let you enter without a password. Did you configure any external authentication providers like LDAP? (not sure if those were added in 9.0.1 or 9.1)
If you continue with the problem I recommend you to open a support ticket and get it thoubleshooted.
As João Rosado stated, the behavior originally reported here is not the intended behavior. The OutSystems Platform should not allow you to authenticate using an empty password.
What happened was that Brian (unwittingly) uncovered a security flaw in the OutSystems Platform. Please refer to this Knowledge Base article for further details, and a fix for the issue.
Thank you for the follow up and communication to users, Ricardo. It's great to see things like this fixed so quickly. Had I realized this was an issue that didn't stem from a simple misconfiguration on my side, I would have reported it in a more private setting. Please offer the team my apologies for the unintended public disclosure!