Fix this.value override to allow empty string

The check for a given `value` was oversimplified and didn't allow for `''`.

I didn't use the shroter form `null == value` because that isn't used anywhere else, but would you consider using that check (there and elsewhere) when looking for `undefined` or `null`?

From [this SO question](http://stackoverflow.com/a/21273362/8279)
> Being a more experienced JS developer now, I should note that using `if (variable == null)` is in fact the standard way to catch null and undefined simultaneously. When writing professional JS, it's taken for granted that the behavior is understood.

Also: http://stackoverflow.com/a/15992131/8279