Allow the user to pass undefined and nulls as scale props
Allow the user to pass undefined and nulls as scale props

This change fixes the problem introduced in 0.5, when the empty scale props were treated as existing values (worked well with undefined/null before).

uber-common/react-vis,uber/react-vis,Apercu/react-vis,Apercu/react-vis,uber/react-vis,Apercu/react-vis,jameskraus/react-vis,jameskraus/react-vis,uber-common/react-vis,uber/react-vis