Email address unique or primary key?

I am a newbie in databases. I read around and found out that it's probably not a great idea to use email address as primary key because string comparisons are slower which effects performance in complex joins and if an email changes I'd have to change all the foreign keys which requires a lot of effort.

But if my users table requires every user to have an email address and each of those email address should be unique, will adding a unique index on the email column suffice? Because afaik unique fields allow null values, whereas I require every user to have an email address, not allowing null values. Is there something I'm missing here? Or I'm suppose to make email column unique and make sure during data validation on the server that user does enter an email address so that every user has one?

PS - Sorry if it's a very basic question. But I just had to clarify this.