Dears...

for a certain circumstances can we let any user to login to pppoe server where the secret is empty in ppp ?

You can probably get it working by using an external Radius server like FreeRadius and have it hard coded to accept any password entry as valid. It's a little tricky, but it is possible to get FreeRadius to do so. Being able to do it with just what is built into the MikroTik is very doubtful.