events: _events to object and undefined not null

By making sure the _events is always an object there is one less check
that needs to be performed by emit.

Use undefined instead of null. typeof checks are a lot faster than
isArray.

There are a few places where the this._events check cannot be removed
because it is possible for the user to call those methods after using
utils.extend to create their own EventEmitter, but before it has
actually been instantiated.