Change AtomicReference#initialize to accept false

Currently, `Rubinius::AtomicReference#initialize` will silently discard the desired initial value for the reference if it is `false`. If one opts not to provide an initial value for the reference, it defaults to nil. In this case, the `#initialize` method will not set the value of the AtomicReference to `nil`. However, rather than checking if the initial value argument is `nil`, it checks if it is falsey. This means that both `nil` and `false` values are excluded. This patch modifies the method to explicitly check if the value is `nil`.