Return an empty list when the requested node is not in the tree

This change makes it so a request for the data in a folder that doesn't
exist always returns promptly. Prior to this, all requests would hang
indefinitely instead.

This is only a stand in for a future solution to send a null result
through the media framework instead. Unfortunately, our structure needs
some tweaking to make that work. This will be fine for the interim.

Bug: 148604797
Test: Build, flash, browse deep into a tree, switch players to invalid
that parents of the node you're on, try browsing upward. See that we
return an empty list for all missing nodes.

Change-Id: Ifd5ed106d3b46377f0d7cd2d1e565cde45ab06f9