Is it possible to have an "unset" contract type?

What I'm trying to do is a simple way to allow a contract to be configured to forward to another contract (dynamically). Something like this:

interface BarOverride {
    function getBar() public returns (uint16 _number);
}

contract Foo {
    BarOverride private barOverride;

    function getBar() public returns (uint16 _number) {
        if (barOverride != null) {
            return barOverride.getBar();
        }
        return 123;
    }

    function setBarOverride(address _address) public returns (bool _success) {
        barOverride = BarOverride(_address);
        return true;
    }
}

So here we have a contract that returns 123 anytime getBar is called, but it's possible to "upgrade" it and set a new contract to handle the getBar call, so that a new result could be returned int he future. But since Solidity doesn't have null, I don't know how to express the barOverride != null part.