android: fix bug with empty directories
android: fix bug with empty directories

Empty directories were not "navigatable-to" (given their empty
contents). However, not showing anything made it appear the
UI entry was unresponsive.

This fixes it an allows the UI to show an empty list of files,
as expected.

Change-Id: Ie07ee72a89ad8ddff139c77d258a04159ffae217

upspin/android