I don't know about every Linux distribution but Kubuntu and all other distributions that I tried always require you to set non-empty password during installation. This password is required when you want to do any task that requires root privileges.
I've mainly used Windows before and user passwords can be empty there, this is convenient if you trust all the people who have access to your computer. If any program requires administrator privileges (analog of root) then Windows just shows popup and you can allow or disallow this program to get these privileges.
Why is the same not possible on Linux? For me, it looks like you are supposed to use Linux in some hostile environment where all your neighbors are trying to get access to your computer.
I always use simple one-digit password to not forget it.
Edit: Everyone and everywhere say that having no password on Linux is very bad idea, it is unsecure, etc. But Windows users generally don't have passwords. I'm using Windows for many years and it never caused any problems for me. I just want to understand why this is not the same on Linux. If there would be no password on Linux then I will still need to use sudo and click Yes when any program asks for root privileges. This way no program will get root privileges without my permission. This doesn't seem less secure for me. It only would be unsecure if there are some evil people around you who have access to your computer and want to damage your system, but this is usually not the case.
Please don't downvote, I'm not trying to start flame war about Linux vs. Windows or something. I just want to understand why things are made this way.