Set a no-op functor when none is provided
Set a no-op functor when none is provided

Instead of having later checks to check for
none when calling the thread bundle callback
just initially set it to a no-op functor that
does nothing at binding time instead; this avoids
the need to do repeated checks at start/stop
time.

Change-Id: I5ab4f801bf4767c802ea607fdf864d4852e6c84d

openstack/taskflow,junneyang/taskflow,jimbobhickville/taskflow,pombredanne/taskflow-1,openstack/taskflow,jimbobhickville/taskflow,junneyang/taskflow,pombredanne/taskflow-1