Initialize scoped boost threads to empty thread instead of null.

The move away from using NULL is that a scoped ptr of NULL being deallocated
will throw errors when in debug mode and compiled statically. After looking
into the reasons, it is best that we just use empty boost threads all the time.

Change-Id: I7567bf45ac298421aa02ca55f21e227e43bfefac
Merge branch 'safer_scoped_threads'

Change-Id: Ibb2d469df2cc62e47d1300a983c8666f7d8e8a25