BindCallback - make it optional to return the listItem

[g105b]

The docs specify that the bind callback must return the listItem, allowing you to manipulate the contents before it's used.

This is true, but isn't enforced - meaning that if nothing is returned, the listItem will be set to null, and therefore no automatic binding will occur.

The callable should be made to return array|null. If the array is returned, use it as a BindableKVP. If null is returned, keep the original, assuming no changes have been made.