Changes null and guard check in _.initial to make style cohesive

In _.first and _.last, the check the style is:

    (n != null) && !guard 

But in _.initial, it was the the opposite (DeMorgan's law):

   (n == null) || guard

When reading the code, this was a little confusing, so I figured I would change it to make the style the same...it could also be switched the other way, but I think this is more logical.