Due to Windows policies, a local system account is limited to using an empty password to console logon only. This is a security setting within the Windows OS and determines whether local accounts that are not password protected can be used to log on from locations other than the physical computer console. If enabled, local accounts that are not password protected will only be able to log on at the computer's keyboard. This implies that connecting by NoMachine with an account having a blank password is not possible unless you configure your Windows to open the appropriate policy.

We always advise to set a password for the user's account connecting from remote. 

If you need to configure Windows to allow remote users to log on without a password, please refer to the official Windows documentation for instructions.