Add preserveNull and preserveEmptyArrays as $unwind options

There are many cases in the BI-Connector (and presumably in customer usage) where the distinction between empty and null array fields in an $unwind must be distinguished, e.g., when mimicking SQL left joins using $unwinds.

Adding these two options would allow for easy distinguishing. For the BI-Connector, we really only need preserveNull, but having the symmetric preserveEmptyArrays could utility.