Replace some options with nullable vars (backport #2658) (#2659)

* Replace some options with nullable vars (#2658)

Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit ac460bfe

)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Fix backport conflicts (#2661)

Co-authored-by: Zachary Yedidia <zyedidia@gmail.com>