unblock list optional tensor

Summary: We dont have an unboxed value to point to in the values table. Hack it to just use nullptr to indicate. I dont love this because its weird to couple the type and the boxed list like this but its probably ok for now

Reviewed By: guangy10

Differential Revision: D48339367

fbshipit-source-id: d9346b072530c3cbda8240087f01becdd4757207