Added simple null validation checks.
Added simple null validation checks.

adamfur/NEventStore,chris-evans/NEventStore,marcoaoteixeira/NEventStore,nerdamigo/NEventStore,AGiorgetti/NEventStore,jamiegaines/NEventStore,D3-LucaPiombino/NEventStore,paritoshmmmec/NEventStore,gael-ltd/NEventStore,NEventStore/NEventStore,deltatre-webplu/NEventStore