If you are sure that you didn't put a password when the first time you created the wallet then I think there are reasons why that wallet is asking for a password.
You might be generated a new wallet before but with a password and you just totally forgot it(Sometimes happen to me even I know that I didn't put a password.).

Or possible the password is not from the Bitcoin core?

Are you sure that you forgot the password? If it doesn't have a password or it's not encrypted you can able to open the wallet.dat file directly with notepad. So try to look for your wallet.dat file and open it with a notepad. If it's not encrypted you should see your addresses and private keys.