Store arrays containing null values as sparse arrays across the code

[dongxinEric]

Right now in insertElementRecursive(), DocLayer does NOT try to store arrays with null values as sparse arrays, i.e. for null value it still insert a k-v for it, which is kind of against what it attempts to achieve later when doing update (skipping the null values and effectively store those arrays as spars arrays). We should make the behavior consistent and do the same in insertion.