If this parameter is included, the Loc will continue to execute even if
currentValue is not defined.
By default, Lift will determine that a Loc does not match a given request
if its currentValue comes up Empty, and as a result will return an HTTP 404.
For situations where this is not the desired, "Not Found" behavior, you can
add the MatchWithoutCurrentValue LocParam to a Loc, then use the IfValue
LocParam to define what should happen when the currentValue is Empty.
For example, given some class Thing, you could do the following to trigger
a redirect when a Thing with a particular ID isn't found.
Menu.param[Thing]("Thing", "Thing", Thing.find(_), _.id) >>
IfValue(_.isDefined, () => RedirectResponse("/page/to/redirect/to"))