Question

Imagine you have a collection of objets and you use the Visitor pattern – how would you handle null retrieved from the collection most elegantly?

Answer 1

Either your problem requires some special type of visitable objects or you’re trying to impose too many responsibilities on your visitor pattern.
If you really have meaningful object which must not be visited try Null Object pattern.

public class NullElement implements Element {

    public void accept(ElementVisitor visitor) {
        // noop
    }
}