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 try to check and maybe you have two wallet.dat file on Bitcoin folder..

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.

That's unfortunate, yes there are some ways to brute force your wallet, only if you knew and remember partial/some details of the password, but on your case seems you totally forget about it, try to remember it, coz bitcoin core will not require and ask a password if you did not set a password on it AFAIK (correct me if I'm wrong though).

BTW this should be moved to Bitcoin Technical Support to get more attention from devs and people who might try to help.