Need better support for arrays with missing members

The MoArray class is used by morpher writers to adapt the sequential array abstraction
to the backing key-value store. There is an assumption that the array members are all
present, so indexes 0-9 means that there are 10 keys in the backing map.

There is a valid use case where array members can be missing. So seeing an index of
9 does not mean there are 10 members - it means there are at most 10 members. So
the concept of array size needs to be full (ie, return 10) even if there are missing
members. The getAt() method needs to return null for missing members.