Allow storing `null` with memory cache adapter.
Allow storing `null` with memory cache adapter.

UnionOfRAD/lithium,pixelcog/lithium,datachand/lithium,davidpersson/lithium,jails/lithium,davidpersson/lithium,graphis/lithium,wbeater/lithium,m4rcsch/lithium-core-library,graphis/lithium,muyehub/lithium,jails/lithium,datachand/lithium,whowgames/lithium,whowgames/lithium,UnionOfRAD/lithium,wbeater/lithium,pixelcog/lithium,m4rcsch/lithium-core-library,muyehub/lithium