Avoid null reference on cache read

[epNickColeman]

Handle nulls when unpacking Equinox.DynamoStore.Core.Token.isStale

[bartelink]

Thanks, not sure how I convinced myself that was not the problem!

[bartelink]

@epNickColeman The expectation of any valid StreamToken is that the .value is non-null

On reflection (OK, on eyeballing the code with sufficient sleep in the cold light of day), the technically correct and/or complete thing to do here is to have all Unpack implementations validate this precondition, and throw an ArgumentNullException if passed an invalid value.

The fault is hence with the caching logic - it's generating invalid StreamToken values and then passing those to isStale. The test stub impl of isStale was not inspecting the token at all (which is something that should be covered...)

Will adjust this in the context of #403