Anti-if purposes: How to check nulls?

I recently heard of the anti-if campaign and the efforts of some OOP gurus to write code without ifs, but using polymorphism instead. I just don't get how that should work, I mean, how it should ALWAYS work.

I already use polymorphism (didn't know about anti-if campaign), so, I was curious about "bad" and "dangerous" ifs and I went to see my code (Java/Swift/Objective-C) to see where I use if most, and it looks like these are the cases:

1. Check of null values. This is the most common situation where I ever use ifs. If a value could possibly be null, I have to manage it in a correct way. To use it, instead I have to check that it's not null. I don't see how polymorphism could compensate this without ifs.
2. Check for right values. I'll do an example here: Let's suppose that I have a login/signup application. I want to check that user did actually write a password, or that it's longer than 5 characters. How could it possibly be done without if/switches? Again, it's not about the type but about the value.
3. (optional) check errors. Optional because it's similar to point 2 about right values. If I get either a value or an error (passed as params) in a block/closure, how can I handle the error object if I just can't check if it's null or isn't?

If you know more about this campaign, please answer in scope of that. I'm asking this to understand their purposes and how effectively it could be done what they say.

So, I know not using ifs at all may not be the smartest idea ever, but just asking if and how it could effectively be done in an OOP program.