Unchecked boost::optional usage in DocumentSourceInternalSetWindowFields

This line in DocumentSourceInternalSetWindowFields::doGetNext() calls the [] overload on a PartitionIterator but then calls the get() method on the resulting optional without checking if it is boost::none. The [] overload can return boost::none, however, resulting in access to uninitialized memory and potential crashes.

The simplest case I found where this happens is when there are no input documents to the $setWindowFields stage.