sharding with empty inner chunk and index location start

[brokkoli71]

In sharding, empty inner chunks get marked by setting nbytes and offset to -1 in the index array. When using index_location="start" the size of the index array will be added to the offset.

This PR will disable incrementing the offset for empty inner chunks (thus, it still contains -1)

may contribute to #2302

[jhamman]

@brokkoli71 / @normanrz - can we close this now.