Cache table entry may be null. Let's not assume it's always non-null.

This is done intentionally because we do need correct table index when
resolving external reference. This requires we do need to allocate array
with the same sheet size as the remote document.  But we don't allocate
Table instances for remote sheets that we don't reference, to save
memory.

Change-Id: I27fb6228f0e4558327aa4a04a6bccce8d2f1085f
Pass source and destination shell IDs through myriad of abstraction layers.

Change-Id: I92e4507857ca64ff6bff31947f2604a4b7c29119