PR #5706 introduced a bug in the sparse array-backed field data

When we load sparse single valued data, we automatically assign a missing value to represent a document who has none. We try to find a value that will increase the number of bits needed to represent the data. If that missing value happen to be 0, we do no properly intialize the value array.

This commit solved this problem but also cleans up the code even more to make spotting such issues easier in the future.